Lime rock is an awesome track, try to make it there 2-3xs per year. Great for spectators, roam the paddock, watch the teams prep the cars, see the drivers in person. Loved going to the alms events some years back, watching lmp1 blast thru the track, nothing better. I was at the Saturday events for the pwc. Lots of fun! Gts is a great class. Lots of great spots around the track to catch great action.
You're likely thinking of GTD. IMSA, with the Weathertech series, uses 2 car categories for their GT racing with GTLM and GTD. GTLM is a modified version of the GTE rules and GTD is a modified version of the GT3 rules from the FIA. They put themselves on a path to do this in order to get more teams from Blancpain, WEC, and other GTE and GT3 seires to just hop over with very few car modifications and simply race in the IMSA Weathertech races. It's why this year, where the GTD rule set was finally fully changed, saw a massive surge in cars for the endurance events, mostly with Lamborghini teams from Europe.