If you want to keep your DIC you will need the metra kit. Like solidhadriel said, it will not allow you to get a DD headunit. So most people just relocate the metra kit in thier glove box so they can get a Double Din (full sized) DVD/Navigation Unit.
crutchfield is a great place for head-unit shopping and has the greatest customer service i've ever used. shoot them an email if you have further questions.
2006 G6 GT
Mods so far: Black arrow overlays, Chrome recessed grilles, Blue LED license plate light, Blue LED reverse lights, Smoked tails, 20% Tint all around, Red led's for the interior, White led dome and map lights.
Sound System: 4 Infinity Kappa component speakers, 2 Amps (950w total) and 2 10" Kenwood KFC subwoofers, 7" touch screen Pioneer DVD
Performance: Mobile 1 oil, Mobile 1 oil filter