It's a weird stance to want our most proven goalscorer to stay in the team? Fwiw I'd play both him and Mitrovic. Rafa will play one of them, and it'll mostly be Gayle imo.
Who's scored more in a single Premier League season? Mitrovic.
Whos scored more in the Championship/League One/Belgian/Serb League. Mitrovic.
So your point is what? We shouldn't play Gayle because Mitrovic has more goals in Belgium?
For the hundredth time, I'd play them both.