A practical option is to use a camera grip repair approach like the DIY methods commonly used for replacement rubber parts, rather than just guessing at a household glue. If the original rubber is still usable, reattaching it with an adhesive intended for flexible materials is the usual idea; if the grip has stretched or deteriorated, replacing or rebuilding it may work better.

One material specifically suggested is Sugru, a self-curing silicone rubber. It can be used either to rebuild a worn grip area or to add new grippy material where the original rubber is failing.

So the short answer is: use a repair method/material suited to flexible rubber parts, and consider Sugru if the original grip material is too far gone. If you can source replacement Canon grip parts, that’s often cleaner than trying to force old stretched rubber back into place.