Fair question.
Im stealing the very basic definition that I think I remember, basically a players' contribution to a win across the board of what he has to do during the game and then aggregating that total for the season/career.
So GMac is going to benefit and be challenged by being on better teams. Meaning, he had Melo and Hak et al vs JGIII having guys that weren't all americans next to him. So GMac is going to have more wins/better production as a team to get win shares from, but he also has better players playing with him to get a piece of that win share.
So MJ would likely be getting every win share in almost whole for any game that his team won in your scenario, but would be limited by total overall team performance because MJ ain't beating the Bad Boy Pistons in that scenario.
Now, is this a fully beloved advanced stat? No. It has a bias built in to players on good teams. Now, you may say "well MikeSU02, of course there is a bias towards players on good teams because that player is a reason that team is good!".
This was a definition on
reddit from a while ago that gets into the nuts and bolts that may help give clearer info.
So, looping back to GMac v JGIII - I think it is more of a "this reiterates what the eye test is showing me". I think it is fair to say that if you just straight swapped JGIII and GMac, GMac would have a harder hill to climb vs JGIII who would all of a sudden be shooting wide open three pointers.
So, this is what I would say:
1) GMac vs JGIII is probably closer than we would generally say based on memory and current impressions that we may have some bias against (I know I basically want to forget last year, for example).
2) I think it's safe to say GMac had more in his bag and ability as an overall basketball player than JGIII has.
3) I think it's fair to say that JGIII probably has never been able to showcase his full potential on the teams he's played on, including this one.
Net net - I'd say GMac is more of a winning player and a better basketball player, but JGIII, while probably more limited than GMac, hasn't been teed up to showcase his abilities in the best way possible.