Cohort commander?

[Flavivs Aetivs]

The Goths of Theodoric alone could field around 20,000 men (estimated), which is larger than the Gallic Field Army of that time.

A Gallic field army under Roman occupation? If you mean in Caesar's time? You looking at roughly over 50,000 and higher numbers that Vercingetorix deployed in a field at one time ranging roughly over 90,000.

And correction, Theodoric's army consisted of 30,000.

Franks maybe, but they also consisted of Burgundians and Alans and Saxons to the north of Roman Gaul.

That's enough money for 40,000 infantry or 18,000 Cavalry per Annum.

That's exactly the estimation of Flavius Aetius's army against Attila.

First of all, consider the fact the Romans weren't loosing. That mean's there a LOT LESS men that have to be replaced and trained.

The size of the Army Aetius brought against Attila is unknown. We can assume that Aetius replenished the Praesental army of Italy with the units Bonifacius brought from Africa after Rimini. With 10,000 men in Italy (or about 1/3 - 1/2 forcelimits based on the size of Stilicho's Army of Italy), plus the Gallic field Army (also about 10000 men) Aetius could have fielded around 20000 men to fight the Goths.
Vitus (although dead in 451) had a "sizeable force" in spain, so let's assume 5 legions for Spain (or about 5000 men)
*(Note that it would be 2 legions if we're referring to Limitanei who numbered estimated 3000 strong per Legion).
Add Marcellinus' army which was also "Sizable" if we assume his claim that he could overthrow Ricimer in the 460's is correct than that's another 5000-10000 men.

Illyria and Spain were tied up, Marcellinus was (theoretically) ravaging Hunnic Pannonia while Aetius brought together the 2 Praesental Armies of Gaul and Italy. With 20,000 men Aetius was facing an Army of around approximately 50 - 60 Thousand men. Aetius had Avitus get the Visigoths (who could field an estimated 20,000 men) (Heather, Fall of the Roman Empire/Hughes, Aetius: Attila's Nemesis) and he negotiated with this supposedly Adopted Son-in-Law in the Franks, which gave him the Salians and Ripurians described by Iordanes. Add that to the Armoiciani (Roman Rebels) who also saw common cause with him and the Burgundians who were bound by a Foedus after being quintessentially Wrecked by Huns and Aetius in the 430's. Also the Saxons probably refers to the Frisii who were settled in Mordern Day Kent and Flanders in the 4th Century, and were migrating there from Holland and the Netherlands due to Marine Transgression. They were replaced in that area by Saxons in the 6th and 7th centuries.

All in all Aetius probably fielded the Remnants of the 2 Praesental Armies, 20,000 Goths, and about 10,000 other Against Attila.

Attila fielded probably around 30,000 Huns based on the Size of the Hungarian plains and the number of horses each rider needed (Heather, Empires and Barbarians) plus about 15000 goths and 15000 Gepids, Franks, Alemanni, Thuringians, and any other tribe that had a bone to pick with the Romans.

Therefore you cannot explicately state that Aetius had an Army comparable to that of Caesar. Gallic Army does not necessarily refer to an Invasion Excercitus. The Roman Army of the Late Era was a different Organizational System that was inarguably more effective in dealing with the Threats presented to them.