Mainly the 1980s and 90s. I love the Le Mans prototype racers, particularly the Group C cars and the LMPs. It's just a shame that Porsche and Ferrari aren't in GT - I don't care overmuch about a missing F355 or 944 or whatever, but I would dearly love to have the Porsche 917, 935, 911 GT1, 908 and 956 in the game, as well as the Ferrari 512s and Ferrari PBs. Lovely cars.
Other than those, I can't really think of many LM cars which should be in GT but aren't. There are some odd additions (the Minolta Toyota - all-conquering in the game - wasn't competitive in real life as far as I know, and neither were the Nissan Group C cars) and I suppose it'd be quite nice to have things like the Courage C49 and C52, and the Dome-Judd thing that Jan Lammers drove recently (that is one of the most bizarre things about GT4, IMO - they put in manufacturers and sometimes whole countries just for the sake of one car, like Spyker or Gillet. Dome and Pescarolo are already in the game, so why not put a few more of their cars in, rather than a whole manufacturer for one car not many people use?) but on the whole I ain't complaining. The endurance races have gotten steadily better and better since the early GT games - I'm looking forward to when we can have bigger fields. Having a Le Mans 24 Hours with a 30-car field would be sweet, especially if they could do a GT1/GT2/GT3 thing, where you could enter - and win - in varying classes depending on your car, winning different prizes depending on the class. That, to me, would be the Holy Grail of motor racing sims.