SSA is their original combat trigger. Moderate pull weight, highest quality materials and process.
SSA-E is a version of that trigger with a significantly lighter second stage.
G2S is the new name for what started as the S2S or "super two stage." Intended as a lower cost version of the SSA design. If I remember correctly the steel used is more ordinary steel than the special tool steel for the SSA, and all machining is conventional - no wire-EDM machining of the sear surfaces. There's also a difference in how it is retained in the lower, but that's of no significance. I think it's nominally intended only for OEM use, but is available through some channels anyway (G&R Tactical is the one I know of).
I've been using the SSA and S2S (same as G2S) for a while. The SSA is a nicer pull, but the difference is very, very small. That's it, practically speaking. I also have a SD-E (flat bow version of SSA-E) which is terrific, but light to the point I would see it as mostly a precision trigger, not an all-purpose carbine trigger.