Seeing as Slot is blessed with two extremely good players his decision will depends on the profile of the opposition right-back their overall tactics as well as the individual fitness or either player. All else being equal, I’d argue that Gakpo is in better form. He also has stronger chemistry with Salah and is more effective at getting on the end of his crosses, which can be crucial. He's also just better at getting the ball into the box.
That said, it’s worth noting that Diaz has been shifted into central areas, much like Gakpo was last year. Therefore, it would be unfair to judge him too harshly as a striker, given that he’s only filling in because Nunez has been underwhelming. It's probably time to drop that experiment now. It looked to work well initially but more recently I've felt Diaz has been badly isolated in the centre and he hasn't taken the chances that have come his way.
I'm confident Slot will do that. He seems very keen on players having their set position and will only utilise any adaptability when he has to.