GP2 to GP5 difference is the length of the flaps, or the wing piece as we call it.
The wing piece is useful when cruising or climbing hills, and provides numerous palm rest positions. I personally recommend at least GP3 and above. I haven't wear gloves after installing Ergon grips. In fact, I was the only one who didn't wear gloves during round island rides cuz I installed Ergon grips.
What is amazing is that even with sweaty palms, or when its raining, the grip is still firm enough and won't slip off.
As for size, its simple. Unless you wear thick gloves during winter (which is non-existent here), just stick to S instead of L. e.g. GP3-S, GP4-S, GP5-S.
And if your bike uses trigger shifters, just use the longer version. If you use grip shifters, you will need to check which side is shorter. Or if you are adventurous enough, just cut the grip and then use a lighter to burn away the burrs.
I always tell people who first buy folding bikes to change their saddle and grips if they are feeling uncomfortable. Everyone who has tried my Tern and Brompton bicycles have all changed to Rido saddle and Ergon grips.