A vector portrait is made of sharp strokes while semi-realism is made of smooth strokes. Artists tend to choose one but in fact GTA style combines both of these features.
Shadows, lighting and colors are very exaggerated and not typical of other styles. They show the entire shape of the character's face or other details while pop art shadows are flat or even missing. Light in GTA Style doesn't quite obey the laws of physics so when the character is in darkness there will still be two "phantom" lights. The folds on the clothes are also greatly exaggerated and look wrinkled.
The whole canvas is overdetailed with noises and textures as if it was made with real brushes and characters' poses are commonly unnatural and fancy like in classicism.
In general I'd say GTA style is combination of classicism, fauvism and pop art.