The GBC setups I've seen/participated in, have included modules using all kinds of electrical components, but mostly Power Functions (PF) since it's not hard to power those with a train regulator that can easily be attached to the power grid.
Recently I've also seen some Powered Up (PU) controlled modules in a setup where most modules where PF.
But as you say PU modules require a bluetooth device nearby, and that gets impractical quickly. Some modules can share a motor, and probably a bluetooth controller, but I don't foresee the guys I know who has 100+ modules buying 100+ phones/tablets. I also don't know if it's possible to make several hundred bluetooth connections within a setup.
As it is now old parts (the train regulator probably takes the lead) are the best for GBC, but that's only good for a limited time (and mostly for those of us who already has a supply of those parts). But I have no idea what the future for GBC will look like (I'm fearing it will just die due to a lack of recent parts that are usable).
Edit: One of the things I set out to write, but forgot as I got into it: If you intend to exhibit your creations (and GBC modules are boring if not), you should also abide by any local rules your local LUG (or wherever you intend to exhibit) has.