The GT3 cars in the game are strange. They’re all R class cars and most of them can’t even reach 900.
With the Mclaren 12C GT3, the game is telling me to upgrade my car in order to be competitive in an R class lobby. The car is fully upgraded and when I click OK to upgrade it tells me that there are no available upgrades.
Would it make sense to be able to upgrade the GT3 cars to 900? Would it be awesome to have a GT3 League Lobby? All signs point to yes? 
Click race anyway. If you can compete with the setup don’t worry about it.
Pretty sure the DIVISION requiremetnts for GT cars is 865 and below at least it is in the career. So not sure about leagues (mp) but I assume it would be the same.
Seems like none of the known good tuners are paying any attention to that yet though- so it’s making me have to get better at tuning so I can still run pro/unbeatable in the career. (Stock is just too under powered for me against the drivatars)
@ OP, Possibly build a 865 or lower PI car and see if you can get in.
On the C7R I was able to have all restrictors taken off in engine up grades , race brakes, and two upgrades in rear tire width with some heavier rims. To get it to 865 — or do it with the car of your choice.
If its 865 and below in career then its possible it would be the same in leagues and lobbies. Probably too early for a GT3 lobby since the game just came out.
Things like this are why I really miss custom public lobbies. Without them things like this, being able to race GT3 only online is either impossible or only available if Turn 10 decides to run a special Hopper for it for a limited time.
The game is great, but not being able to log in and do the type of racing I’m hoping to continues to be Forza’s biggest drawback, IMO.
I’ll vote for a GT3 only lobby.
As for racing in the R lobby with a <900 car, as pointed out earlier, click “race anyway”. I did that with lower rates cars when I was racing against beginners as a >1000 level racer in FM5. My 656 Transit loved beating 700 Ferraris.
A seperate lobby for gt3 cars would be amazing.
On a side note, does anyone know any good tuners that focus on gt3 cars? Its been really hard to find good tunes and id rather not pay for 20 different tunes until i find one. Seems like the ones i try (that are highly rated) fishtail and oversteer into corners. I dont think its me, since i have driven good r class tunes for other cars without that problem.
I tune most of the GT3 cars and mostly play R class in the lobbies. I play with TCS and STM on. If you use my tunes without TCS and STM you’re definatley going to oversteeer.
A GT3 endurance lobby and league would make my dreams come true. There’s so much variety in GT3 cars and with a rating cap at 865 it would be very balanced.