I think the G2 is the revised, newer version of the Guide. I don't know what SRAM changed for the revision.
The Level was a lighter brake. I think the Level was marketed for XC and the Guide / G2 for trail. Guide / G2 has 4-piston calipers, and I think all Level use 2-piston.
There are / were different variations within each, like R, RS, RSC - I don't know if any make any real difference in stopping power.