Gomez is good at his strength. He is a very physically powerful player still with a lot of pace. With space in front of him and running on the ball or with the ball towards one or even two defenders, he is almost impossible to stop. And when he gets past them, he has very good nerves and a strong shot to finish it too. He is also tall and while not having excellent heading skills hes still strong at scoring with his head too.
On the other side his ball control and first touch is awful. He is also hardly agile and both combined makes him an almost useless attacker in and around crowded boxes. You neither cant play the ball at him as he can hardly handle it or (if he actually does so) cant get in scoring position or play a trough pass or anything, just b/c he lacks the ability to bring himself away from defenders into a good spot fast. Nor is he really mobile enough to be good at running into a tunnel for a through pass.
So he is really dependant on the right team. That exactly was the reason van Gaal never counted on him, b/c his strength werent used in his system, where the ball was controlled deep in the opposing half with the opposition mostly defending deep. He rather needs a direct approach with the own team being deep and countering fast. This way he has the space and exposed defense against him. Thats when he shines.