The word gamer is old, evidently dating back to the 1600's. Using the unqualified word "gamer" in place of "video game player" is relatively recent. Also, according to the Google n-gram viewer, usage of "gamer" started ramping up in the late 1980's.
Essentially, the word can be considered a sort of "quasi slang". Its use has increased recently, and at the same time it has acquired a meaning which it didn't have before: the power to express "video game player" without video games having been previously mentioned.
If the gamers being discussed are not strictly video game players, but players of various other indoor games, such as board games, then the word "gamer" will cause confusion, since it will likely be interpreted as "video game player". You should use a term like "board game conference" if it is about board games, and the term "board gamer" is also a possibility.
I would not avoid "gamer" in a formal document, but I would avoid expecting the reader to understand that it means "player of video games". A better idea is to use "video gamer" at least once; after that, "gamer" can be used. An alternative is to use "video game player", and subsequently just "player". Likewise, if you make it clear that the document is about board gaming and use "board gamer", then the word "gamer" later in the document will also be clear. Once you establish your document's own specific context for the interpretation of the word "gamer", it loses its ties to modern slang. The word now means "the type of game-playing person that this document is about".
Players of various sports (not necessarily athletic) are sportsmen, and if they are adult females, they are sportswomen. There is no sports-something word which denotes sporting people that may be men or women, or possibly children. "Sportsperson" is acceptable in a formal document; it's clear that it's a gender neutral version of "sportsman" according to a well-established modern pattern of gender neutralization. I mention "sportsman" because the original meaning of "gamer" was essentially that of "sportsman", when "game" was understood to refer to a sport (including the sport of hunting, which is why the word "game" still refers to hunted animals).