I don't think that's it, I just checked out and the game only takes around 25-26% ram and it was running reasonably well with 4,5gb ram already taken. that 25gb ram sounds like someone is playing the game with 4k graphics or something. bg 3 lists only 8gb ram as mimimum requirement and 16gb is recommended
edit: I don't know if gnome takes up some grpahics card or cpu resources which affect the game performance since it's a lot bigger program than i3wm