The reason to start Gyokeres is because he has momentum, rhythm and has finally built relationships with the team and his record since the turn of the year has been great. He's also instrumental to the press and the number of goals we concede when he goes off is testament to that.
Havertz on the other hand has had a stop start season, has just returned from injury and hasn't put two games together. He's not firing on all cylinders like Gyokeres is. All things equal it would be Havertz but right now, it's a no brainer to start Gyokeres and he will start.