Rubber - did the Romans have/use it ?

[Tarbicus]

What the Romans undoubtedly knew were gum resins, which can serve similar purposes (waterproofing, frex),

A tad OT; Mastic, extruded from the bark of Pistacia Lentiscus (Schinos tree to some), was used for ulcers of the mouth and also as a chewing gum to sweeten the breath. 'Doctors and Diseases in the Roman Empire', Ralph Jackson.
The Wounded Tree
Mastic Is More Than An Antibacterial
King's American Dispensatory
And if you fancy chewing gum during a reenactment event here's a supplier in the UK: Skin Shop (mastic gum). They have other mastic products, too.