First, I'm assuming "The Gasket" you refer to is the one on the bayonet mount.
If that's the case, you can just leave it alone and not worry about it. The weather seal on my 70-200mm wore out long ago and I never bothered to replace it. Never had any trouble shooting with it that way.
However, if you want to do something about it, you have choice of...
- Send the lens to Canon for replacement. It will cost about $40 for the part and labor, plus shipping both ways. (Might be more, that price is from an older quote.)
- Or take the lens to a local camera repair person and have them replace it. They can order the part from Canon and install if for you. It will cost less and, if you can do it locally, will save on shipping.
- Or buy the part and replace it yourself. It's really pretty simple.
Replacement procedure:
1. Remove the lens from the camera or the rear cap from the lens.
2. Use a marker and make a registration mark on the bayonet mount and barrel of the lens, so that you know how it's oriented on the lens and can return it to the same position.
3. Use a good fitting screwdriver to remove the bayonet mounting screws. Probably the screws are "JIS" (Japanese Industry Standard). Be very careful to not slip. (It may be a good idea to cover the optical element with "dental dam" or similar.)
4. After all the screws have been removed, slowly and carefully lift the bayonet mount while looking to see if there are any shims under it at the screw holes. If there are any, the shims will be very thin, small, metal "washers". Also check the back of the bayonet mount to see if any shims are stuck there. If you find any shims you MUST carefully set them aside in an organized way so that you can put them back under the bayonet mount exactly where they came from. There may not be any, but some lenses use shims to precisely align the lens on the camera. If shims were used on your lens, failing to replace them in exactly the same places would mis-align the lens once it's mounted on the camera.
5. Remove whatever remains of the old, damaged rubber ring.
6. Fit the new weather seal ring in place. The newest ones are now made of silicone for better durability than the old rubber ones.
7. Replace any shims as they were originally.
8. Replace the bayonet mount in it's original orientation (which you marked earlier).
9. Put a very small amount of blue Locktite on the threads, then carefully re-install the screws that hold the bayonet mount onto the lens.
10. See the following link to one source of the replacement weather seals (you also can buy directly from Canon... they might even be cheaper that way).
https://www.ebay.com/itm/322477736280