The customary Roman campaign season started on March 23, called the Tubilustrium, and ended on October 19th, called the Armilustrium. Winter quarters existed to not only give the soldiers a much needed break, for officers and commanders to be switched out with new ones, but also out of logistical reasons, as grazing was nil so foraging for feed for mules and horses couldn't be counted on.
Caesar's most active year in Gaul would have been in 52 BC, when he had to put down Vercengetorix's revolt. So let's use that one as an example for this discussion.
In that campaign season, starting in March and ending in September, his army marched well over 1,200 miles of actual roads, not counting day to day movement, foraging, patrols, etc. They fought in many major battles and sieges, Vellaunodunum, Genabum, Noviodunum Biturigum, Gergovia, Avaricum, and Alesia, not counting countless small skirmishes. Caesar's supplies were definitely low for most of the campaign, they lived off the land and the stores captured in Gallic oppidum. As bad as the situation was, it was never as desperate as in Caesar's Greek campaign against Pompey, where his army at times was literally on the verge of starving.
Caesar's army could definitely use the captured food, horses, mules, wrought iron and tools, etc. But I'm not quite convinced that they would have suddenly armed themselves with Gallic weapons just because they were available. Gallic armies were traditionally spear heavy, compared to the Romans using swords for close combat. Gallic swords were traditionally longer than Roman. They didn't use pilum. Their shields were of a slightly different style (flat thureos, not of the same width and height as a scutum, probably not rimmed with metal). Helmets and mail would no doubt have been appreciated, Gallic armor seems to have been all the rage at the time. My guess is that any Roman soldier who might not have already had top notch armor at the start of the Gallic War could have acquired it at that point from the corpse of a Gallic nobleman. Romans didn't wear trousers, long sleeve tunics wouldn't have been desired (most of the campaign was fought in warm weather). Footwear was different as well. So likely Gallic clothing wouldn't have been first on the list of stuff to plunder.
So where does that leave the individual soldier?
They would have been fattened up a little during the 53/52 BC winter quarters in Italian Gaul while Caesar was back in Rome dealing with the turmoil from the death of Clodius. Italian Gaul was downright civilized compared to Cisalpine Gaul and Gaul of the Long Hairs. So they would have access to better food, better wine, more to their liking. But most of whatever extra weight they'd put on would have been lost by the time they went back into winter quarters at the close of 52 BC. They probably would have been very wiry, veiny, in great fighting shape. They wouldn't have looked like bodybuilder or weight lifters, the movie 300 got that all wrong. Ancient soldiers of the lower classes of citizenry would have lean and hard muscles, even though they'd be strong as a bull. That campaign year they didn't seem to have suffered from starvation to any significant degree to have suffered major long term medical problems, so they'd be skinny but healthy. But also exhausted physically and mentally after a long, hard, and extremely violent year. Lots of short tempers and hollow eyes. I would not want to get on their bad side, you'd be liable to take a gladius under the ribs.
Hair wouldn't be stylish at all, very basic haircuts that any camp calo or tent mate could do with scissors. Bowl cuts or close to scalp (for comfort and lice issues). Faces would probably have stubble on chin and cheeks, since shaving daily is too much effort, but no full blow beards, for cultural reasons and hygiene (lice).
Small rents in their tunics would have been sewn, if the men couldn't do it, their camp servants could. If their tunics were dyed, the colors would have likely faded over the year from the sun and from washing. Same with cloaks. The only men still wearing bright colors would be the wealthier officers, who could afford the better dyes and had the room to pack multiple items of each clothing, so as to always have nice looking attire.
Their arms and armor would have had significant wear and tear, but considering that most of Caesar's twelve legions in 52 BC were veteran, they would have had known how to keep their equipment in serviceable condition. Helmets would be fine, maybe a few dents or scratches, but it takes a lot to ruin a helmet without killing the wearer. Body armor would probably in good working order, oiled, shined. Mail, scale, musculata, cardiophylax, or linothorax all would have been rather easy to maintain and any significant damage would also have resulted in the death of the wearer, so maybe a few scratches or dents. Small leather thongs or straps would be everywhere in the ancient world, so finding replacements wouldn't have been hard at all. Like helmets, body armor would have been shined/buffed, as nothing says motivated and disciplined warrior like brightly shined armor.
Weapons would probably be in good order, especially since legions had their own smiths to make field repairs. Though after all that hard fighting over the years I doubt swords would looked pristine. Notches and rolled edges, bent tips hammered mostly straight. Worn but well oiled and adequately sharpened. Casualties would have meant that unrepairable swords could be replaced. Pilum were known to be fixed after battles, so they could make whatever repairs needed. Probably not good looking weapons, rough and crude but easy to fix and balanced enough to throw reasonably accurately and far. A few men might have replaced lost pilum with captured Gallic spears or javelin, those could still be throw and cause damage, just not as effectively as a pilum in piercing shields.
Battles and weather would have put a hurting on Roman shields, they probably wouldn't looked all that great, with holes from arrows and spears, tears and gouges from swords, dents along the rim, issues from water soaking through (like crossing rivers or getting caught in the rain where the shield cover wouldn't help). I seriously doubt whatever shield design had been painted on earlier would still be the same color or as clear as it was originally, they would have been faded from use.
Besides the shield, the boots would probably be the most unserviceable piece of equipment they had. Unless the caligae, calcei, or perone were made of such fine construction and material that they are better than any other military boot in human history, there is no way a single pair could have lasted through a campaign season without having to be completely rebuilt at least once, with significant alterations and repairs needed on a regular basis. I personally think that at least some of the men would have preferred marching barefooted just to prevent wear and tear on their boots, so they could save them for battle (where they would be most needed).
Catching them on the march, the soldiers would have dusty feet, maybe mud splattered legs. Their tunics would be soaked in sweat. Even so, they would not be having any obvious physical issues with the march, walking with full kit and loaded furca would have been as natural as breathing. Maybe singing marching songs while plodding along, one foot in front of the other, hour after hour. They would have found the perfect way to carry everything with the least amount of effort or annoyance, everything in its perfect place, strapped down, nothing banging around in an annoying manner. They would have smelled of sweat and body odor, oil and leather, a bit of mildew, and probably a bit of garlic. Likely some might have developed muscular disfigurements (such as
Indonesian sulfur miners ) along their shoulders from carrying heavy loads on a pole. I'm also guessing some would have some sort of large calluses/calcium deposits on the top of the left wrist and the left elbow, possible the left knee, from carrying and using the scutum, probably a sign of being a long service infantryman (similar to bow legged cavalryman).
