Have original Borderlands installed that I picked up from GOG.
Found wemod while searching for cheats. Installed wemod desktop, and pulled up Borderlands, but the only option I get is to “Install Game”. There seems to be no option for directing it to the already installed location.
Hello and welcome to the community. 
May I ask how you have Borderlands 1 (or Borderlands Game of the Year Edition) game from the GOG store? They do not appear to currently have this game in their store, at least according to my searches there.
If you are referring to a game titled “Tales from the Borderlands”, this game was but is no longer in the GOG store, as its developers (Telltale Games) shut down. (Reference: https://www.gog.com/wishlist/games/tales_from_the_borderlands).
The other Borderlands games were developed by a different company called Gearbox Software.
Either way, since the original (or GOTY) version of the game does not appear to be in the GOG store, there is no trainer here for it. As of December 2021 it is still a user-requested game in their community.
My bad. I had to go back and look. I downloaded two older games (sick day off), and it was Assassin’s Creed (directors cut) that came from GoG; Borderlands came from another gaming website.
So… I’m assuming that unless Borderlands 1 is installed through Steam it won’t work with the WeMod App?