I've tried all kinds of tech cards in G&G this season, I started with Shiftry Ex (CG), Furrett, Ray Ex d (DF), and a Holon Castform. I've also used Gardevoir Ex d, Unown G, Blastoise d, Dusknoir, Palkia Lvl X, Cresselia Lvl X, Claydol, Mismagus, Banette, Swampert (GE), Crobat, Absol (MT), and Sableye (CG).
Depending on the build, G&G has anywhere from 2 to as much as 8 open spots for tech cards. Typically, though you will see about 4 or 5 cards used for tech purposes. There are still other cards that can be easily teched in I haven't tried yet, such as Dugtrio (CG), Lucario and Lucario Lvl X, Feraligatr (SW), Porygon 2, Lunatone and Solrock, Gardy (PK), Gyrados, and Xatu.
Basically, since G&G is really a single line of pokemon and you use DRE and Scramble, you could literally fit anything into this.