Prior art is irrelevant for trademarks -- a trademark doesn't have to be unique for you to use it. There was an item called a "macintosh" before the name Macintosh was trademarked. The word "dropbox" had been used in file sharing software long before the YC startup existed. Even the creator of gaymer.org doesn't claim to have invented the word "gaymer."

You can claim a trademark as long as no one else has already claimed the same name in the same context. Once you have established the trademark, you can prevent others from using that name, even if they have been using it for a long time.

Note that the "same context" is why there's unlikely to be a conflict with Gaymers Cider. In particular, the gaymer.org trademark only applies to social networks.

(IANAL, so I may have some details wrong here.)