On the Geo series:
Geo1 was shit. It relied on a silly ball check-valve in the solenoid to seal the firing chamber, and apart from that it was a standard dump valve marker. It's shit.
Geo2 is great. Actually has a moving prop shaft (or whatever its called) to shut off the air flow while firing, very similar to the old matrix slambolt. Definitely worthwhile innovation.
Geo3 was basically the exact same as the Geo2 except it now has a fitting in the trigger guard to control for much air exhausts. Not worth it in my opinion, but some people think it's great.
The rest of the Geos are all minor updates and not independent markers. The GSL is NOT a Geo4. Planet Eclipse has done this with the Ego SL series since 2006. The entire idea is a limited edition marker with all the bling features you can think of like full barrel kit, multiple triggers, crazy gold-like plating on screws, crazy milling, etc. It's target audience is most likely not you. The Geo+, Geo2.1, and now Geo 3.1 are also just minor updates because they don't change the Geo that much. The changes are largely cosmetic and ergonomic (and I greatly value ergonomic changes).
1) there have only been 3 distinct Geos
2) Only the Geo2 and beyond are actually worth anything
3) PE isn't Dye that re-releases the same marker with different milling every year.