This car is possibly a Dewbachee car, and the only car proir to GTA V is the Super GT. In GTA V the Super GT is based off the Aston Martin DB9 and a Ferrari 550/430. While this "unkown" vhicle is based primarily off a Aston Martin V8 Vantage and a Ferrari California. Both of these cars are primarily based off an Aston Martin Grand Tourer, with some parts mased off a Ferrari Gand Tourer. I am starting to wonder if this unkown vehicles could be the next Super GT. (insert sig here)
Do a google search for Toyota Celica GT-S or Celica widebody. I have one and this "dewbauchee" has a lot of design features that are similar in the front 1/3. especially the headlights/curves around them, the curve of the hood, coupe style, short wheelbase, the front grille has the same shapes just flatter and larger than the celica,
This car also seems to have some influence from the Nissan GT-R, it also has slight similarity in the name, specifically the "GT"
A quick google search brings up the wikipedia page, with a picture of the car in question. It's got some similarities in the fenders over the front tires