I just downloaded Galaxy after already having a few games off of GOG because it said you could play multiplayer against others, however im not sure exactly how to do this when im on the home galaxy page after i log in i can access my games but it is just telling me to install them, i already have them installed and playable on my computer do i have to install it again to access it through galaxy to play against other people?

Most games do not use Galaxy for multiplayer. A couple of the few that do are The Witcher Adventure Game and Aliens Vs. Predator Classic 2000.

If you have those, you might need to have Galaxy detect your installation. Just about every other game that has multiplayer at all use other services or even direct connections and have nothing to do with Galaxy at all.