The McLaren 620R would be a good addition to the GT series, it makes 612 horsepower and weighs 3,241 pounds, which is similar to some cars already in the series, such as the Bentley Continental GT3 or Ferrari 488 Challenge. It is also RWD, so it would have similar handling to the other GT cars.
Below is a comparison of all the GT series cars power to weight ratio with the 620R included:
Ford Falcon FG X: 0.241 (677 HP/2,814 LBS)
Nissan Altima Racecar: 0.240 (695 HP/2,895 LBS)
Holden VF Commodore: 0.234 (679 HP/2,901 LBS)
Ferrari 488 Challenge: 0.209 (661 HP/3,162 LBS)
KTM X-Bow GT4: 0.206 (496 HP/2,413 LBS)
Dodge TA Challenger: 0.201 (590 HP/ 2,941 LBS)
Audi R8 LMS GT3: 0.200 (585 HP/2,931 LBS)
BMW M8 GTE: 0.193 (551 HP/2,853 LBS)
Bentley Continental GT3: 0.190 (600 HP/3,161 LBS)
McLaren 620R: 0.189 (612 HP/3,241 LBS)
Ford GT Le Mans: 0.188 (530 HP/2,825 LBS)
Acura NSX GT3: 0.187 (544 HP/2,906 LBS)
Porsche 911 GT3 R: 0.184 (518 HP/2,816 LBS)
Aston Martin V12 Vantage GT3: 0.182 (552 HP/3,034 LBS)
BMW M6 GTLM: 0.179 (512 HP/2,861 LBS)
Porsche GT 911 RSR: 0.176 (503 HP/2,853 LBS)
Chevrolet Corvette C8.R: 0.175 (527 HP/3,020 LBS)
Chevrolet Corvette C7.R: 0.173 (491 HP/2,842 LBS)
Dodge Viper GTS-R: 0.173 (496 HP/2,869 LBS)
Porsche Manthey 991 RSR: 0.173 (490 HP/2,838 LBS)
Lamborghini Essenza SCV12: 0.164 (589 HP/3,601 LBS)
As we can see, the 620R would be about average in terms of power/weight in the GT series, and would make more sense as an addition than some cars that are already in the series, like the Ford Falcon, Holden Commodore, and Nissan Altima.