So to kick off I want to list some of the reasons why I think
GK are overpowered (obviously not all of them) (also not in a strict order, but the lower numbers are more important):
1) All
GK vehicles are practically immune to any kind of suppression. A ~92% chance to pass on
Ld 10. This is one of the most powerful abilities in the game. This upgrade comes at +5 points to its Vanilla Counterpart for most vehicles.
2) Psyflemen Dreads. 135 Points for 4 Twin Linked S8 shots at 48" range from a 12/12/10 Walker that also gives massive psychic defense to your army, and easily grabs a cover save from behind a Rhino Chassis. Also it can be used to tarpit enemy units that don't have a powerfist/similar weapon.
3) The mobility and spam-ability of Psycannons. Either you can move and fire (or drive + disembark + fire for flank shots or denying cover) or, if you stay still you can get 4 shots per psycannon. Thats like having 2 autocannons. That rend.
4) Purifiers. Double the psycannons, 2A with power weapons each, and Cleansing Flame. Deadly to armor, deadly to heavy infantry, deadly to hordes. (Get two of these in combat with 30 Orks and you wipe the squad before strikebacks)
5) Psybacks. One of the most cost-effective units I have ever seen. 55 (I take dozer) points for 36" S6 AP4 Twin Linked (that cannot be suppressed)
6) Warp Quake + Coteaz - Immediately makes Daemons,
DoA BA, and similar deep striking armies cower in fear.
7) Being able to have a S10 hammer in each
GK squad for 10 points - Oh, you didn't move that vehicle with 10 rear armor? Charge, 3 auto-pens. Also helps against
AV 14
8) Super cheap suicide units - 54 Points for 6 Warrior acolytes with 3 Meltaguns. Although BS3 they still pack one hell of a punch for 54 points, and bring along an extra psyback.
9) Coteaz for 100 pts is a steal. Near-Immunity to your opponent stealing the initiative is the best reason, but unlimited henchmen, 3-4 Extra Hammer attacks, a good save, and extra hammerhand, Sanctuary, and his anti-reserves ability are all amazing. Stick him with some purifiers in terrain near your table edge and dare those genestealers to outflank.
10) The best choices in the book (Dreads, Purifiers, Coteaz, Henchmen, Strike Squads) all take up different slots of the Force Org. Psybacks are a
DT so can be taken in many slots. This allows the ability to spam all the good choices without being weighed down by slots.
11) Already mentioned these individually, but between Psybacks, Psycannons, and Psyfleman (these names are rediculous

) you get a ridiculous amount of S6+ shooting. This causes
ID for T3 models, can suppress
AV 10/11 forever, is very effective against hordes, and causes anything with 3+ armor to just keep rolling saves. They will fail eventually.