Man, some of the comments here are certainly from people unfamiliar with trademarks.
They are going after you for using the Gorilla trademark in connection with the sale of motorcycle items. This is why it is ok for Gorilla Glue to be fine and other Gorilla items not related to automotive.
You could research their trademark filings to see if they are, in fact, for use on motorcycle products if you would like. Your best bet is to find a good trademark attorney ASAP. Do not answer that letter on your own.
In order for them to hold the trademark of Gorilla, they have to enforce it. This is merely a case of them enacting such enforcement. It has nothing to do with going after your profits, but protecting their brand.
Good luck. These sort of things usually come out to who has bigger, deeper pockets. No matter what, be prepared for this to cost you some serious cash whether that is in the form of a settlement, redesign of your website, lawyer fee, etc, it is going to cost.