Gonzalez is way better than Kostic.
Kostic, for what he does, is a good player IF your system and team plays to his strengths. He doesn't defend, he doesn't score much, he's not fast, he's not good 1v1. But he bombs A LOT of crosses into the box.
Gonzalez is an actual modern winger. He can cross, he is better 1v1 than Kostic by a wide margin, he is faster, he's better with the ball at his feet, and his ability to cut in and shoot is an actual threat opposing teams will have to respect. I struggle to think of anything Kostic does better than Gonzalez. Even defensive duties, Gonzalez is infinitely better in that regard.