Is there a certain forum or do we email them?

https://www.gog.com/wishlist/games

You can vote for a game and if it's not there you can add it.

This. It could not have. been better said.

I'd say that you first convince the publisher to submit it to them, since it likely takes some pretty significant perceived interest for GOG to try to get a game not sent to them, and if said perceived interest exists one more user requesting it in any way would make no difference. Still, in this latter case of GOG trying to get a game the publisher doesn't immediately care to sell here, wishlist votes may help somewhat, if there are many of them. Directly contacting said unwilling publisher is still likely to count much more, however.
On the other hand, if you're talking of a game that was submitted to GOG and GOG rejected, or is still debating whether to accept, then wishlist votes may help, yes, depending on numbers. Messages sent to support may also help, though if they'll reply at all they're likely to just say they can't comment on game additions, because if they receive enough someone might take notice.

On the other hand, if the question is about submitting your game to GOG, the link's at the bottom.