Well, not as many experts here chiming in as I hoped.
Called a few GEM specialists and salvage yards and got this:
General groupings
1999-2004 Generation 1
2005-2015 Generation 2
2016- Now Generation 3
Generation Details:
1999-2004 Generation 1
-2000 - Upgraded from 48 volt to 72 volt system
2005-2015 Generation 2
-New suspension system
-Front disc brakes added
-Polaris bought GEM from Chrysler in 2012:
*2013 - improvements to fit and finish begin
*2014 - T5 controller introduced
2016 - now Generation 3
-Complete new body style
-48 Volt system
Please add or expand any info you may have.
Since final verdict on the Generation 3 is not out yet, it appears the 2014 and 2015 are the ideal GEMs to purchase used