He was much worse against Monaco. It's not that anybody's saying he could never be a very good player, but to be Man Utd level, he'd have to have shown something more than he has in European games against better opposition, because that's the closest we can get to seeing him against PL sides.
Martinez and Antony have put in huge performances against teams like Dortmund, Sporting, Benfica etc, they're not being signed purely based on what they've shown in the Eredivisie. Gakpo hasn't taken the same opportunities against the same quality of opposition when he's had them.