I own Fallout 1 and 2 along with Tactics. Also Septerra Core on both steam and Gog, but even if I uninstall the game? It won't let me install the Gog version. It keeps saying opening Steam is there anything I can do?

right click on a game you own on multiple platforms - hover over the bottom most option that looks like two diamonds overlapping with a number next to it - you can then pick the platform the game will be installed/played from