The Forza GT division is a complete menagerie cars from which our built spec for different divisions of racing
GT Endurance
GTLM
GT3
GT2
The mix of cars and specs of each of them and limited upgrade potential makes a decent amount of the cars not competitive versus cars like the Audi R8 and the Lambo Super which are putting out lap time 5 seconds+ faster than cars like the Merc SLS, BMW Z4 GTE, and Nismo GTR.
My guess is the GT division is the best place for a GT car 
You forgot Super GT (the NISMO), which in real life is on the pace of LMP2 
In any case it’s not a real GT Division, it’s “Forza GT”. Turn 10 probably thought it was best to lump every GT-type race car into a single category than to have multiple similar Divisions. The game already has around 60 Divisions as it is, and it’s best not to go overboard.
The minute you get the career out the way and start tinkering with your own cars that all is blown out the water anyway. My 63 VW Beetle is up against the Bugatti Veyron which I have to let lead for 99% of each race so I can use its width and braking to pull up on every corner. I have been using the Max allowable HP settings to match mine which narrows the field of available cars and even Car Make/Model to hone thing better in standard form cars giving a more fair and balanced approach.
Simply not true. You can make every GT car competitive in the hopper, even the Gallardo which I had originally shrugged off as a wet dud. The Merc, Z4, and Nismo are all super fast and I’ve got multiple top 10 regional times in all three of them. The M3 is by far my favorite.
Every car from the GT Division is competitive. No disrespect, but maybe you didn’t tune some of the cars right. The Nissan Super GT car, which is on pace with LMP2 and DTM cars in real life, and the Maserati Gran Turismo GT4-spec car, which is slower than a GT3-spec car in real life, can be equally fast due to homologation. It’s not realistic, but let’s not forget that this is not a simulation game, like iRacing or Project CARS 2.