Hello.

I've bought both Quake and Blood in GOG store. I'm using different sourceports instead the ones that are provided. My question is: Is it possible to GOG Galaxy's LAUNCH direction from the original one to a different one? (I want to track my time spent on playing).

There is this thread about adding non-Galaxy games to Galaxy (which the source port is, despite the game data coming from a GOG release). That thread does mention that you don't get time tracking, but you can experiment with setting the gameId of your custom file to the actual gameId of that game as it isn't very clear on whether that's only for non-existing gameId's.