What compressible rubber like gasket material is the least permeable to CO2?
Viton is a trade name for a fluoroelastomer type. There are different grades which have different properties. I've had a Viton seal be deformed in a vessel when using tetrahydrofuran as a solvent, whereas Kalrez does not. I've seen different Viton grades again not have great compatibility with acetone. So do check out the compatibility of each supplier's product.
We have two seals that we use for $\ce{CO2}$ applications. EPDM and PTFE.  These both cope with 100 bar pressure.