For your info Gyokeres wasn't having any other English premier league club in mind including Manchester United except for Arsenal. He made that clear which was much reason Arsenal had to increase their efforts in signing him to the club. Moreover, Manchester United isn't playing in the champions league next season which I believed may have made Gyokeres prefer Arsenal to United.
Real ones know that there is nothing left in Manchester United, the name has been made but the team isn't really representing the club at any level. Arsenal hasn't miss the Champions League for the last 4 seasons Arteta has been with the club but Manchester United has been inconsistent all this while. Before Manchester United will be in good form, it will take at least 2-3 years before they get theirselves together and how many years is Gyokeres contract to begin with.
Arsenal is a club of people with long term goal, he can go to Manchester United and might not be able to deliver what they want and if care isn't taken, they can laon him out like they have done with some players. No serious player will want to be loan out to another club when you can clearly represent what you came for. There are endless opportunities for Gyokeres to excel at Arsenal than for him to do well in Manchester United, it's even possible Manchester United might not make it to Champions league for next season.