From what I believe, the released the Geo as the original and was just the Geo. However, this had a few problems, but as always with eclipse they were ratified quickly and they released the GST and Gr2 kits to resolve these problems. Markers with these updated parts were known as the Geo+ I believe.
Then came the Geo 2. This was just the newer version as with the Ego lines, but i can't tell you what changes were made. After the release of the Ego 11, the Geo 2.1 was released, which just changed the components of the marker externally, which included the POPS ASA, 45 swivel on the regulator, and the shaft 4 possibly (not sure if this was already on the Geo 2's.
And now they have the Geo 3. The body has a different design, noticeably the eye covers, and there is a solenoid adjuster on the trigger guard. It also comes with 2 bolts, one is the standard Geo bolt, and the other has a soft tip to be more gentle on paint.
There will be more people who can give you a much better idea of what changed between the different models, and will probably correct me as I'm bound to have made some mistakes with what i've said, but that's a rough idea to the different versions and in which order they came out, hope it helps