It's a version of proton with some of the enhancements that the main proton team either won't or haven't added yet. I haven't looked into whether it's an individual dev doing this on their own or if they are affiliated with the proton team. I run the GE versions of proton because they solve some problems I have in certain games without breaking anything. It's my understanding that the proton devs are from the same team as the wine devs and that they have strict rules about introducing new code that doesn't meet certain standards for wine (i.e. affecting more than just games). The GE builds include some of this code that hasn't been integrated yet "in the right way".

This is all just my understanding from reading various discussions, so may be incorrect in whole or in part.