That's right, the buildings in Gpit are Func_details.
Keep in mind that's not exactly the 'best way' to build. It works in Gpit because each area (Blue, A,B,C) are all one large area, but sealed off from each other good.
Making those buildings world spawn would cut up the vis into a ton of visleafs , and since they are full of windows you'd never be able to seal them anyway. So func_detailing probably reduces the visleafs in the area by 1/4 which is a pretty big optimization.
In that case it's better to have everything render at once then having a million leafs and probably still rendering 75% of everything anyway.
Most maps though are sealed more, have more solid walls, fewer windows, larger buildings. So func_detailing the buildings in Well for example (center point of CP_Well) would be worse for performance.