All 98-08 grand prixs came with a 4T65e. They came with different gearing and different parts in certain years or are "HD" models, but the grand prix itself never got the 4T80e.

Only the bonneville GXP from 2004-2005 got the 4T80e with the northstar motor. Yes others cars got it aswell, but mainly all caddy's and few olds aurora cars with the northstar.