Of course, Donaire will take it seriously as he can't afford to lose the opportunity to have a rematch against Inoue, if he loses, I doubt Inoue would still give him a rematch when he doesn't have the belt anymore, it will be Casimero vs Inoue if Casimero would be successful in defending his belt.
If Reymart Gaballo doesn't get anything in their current training then the serious Donaire will finish him in the early round because he is not his priority right now. This kind of fight before the dream match would be good as well because if he ever sweeps this win like he usually does, the fans want to see him fight the best in his weight class which is Inoue or Casimero. seriously, those two should not be fighting other boxers other than best fighters like them.
I agree, Gaballo needs to have a unique strategy to earn the respect of Donaire. If he fights the way from his previous one, Donaire can easily beat him. This guy is on a mission to regain his throne. He will not give anyone a chance to prevent him from achieving his goal. Gabarlo needs to undergo hard trainings and take some tune up with the fighters that have better knowledge with this sport. He needs to prepare more than he usually does, knowing Donaire's caliber.
And like you said, Donaire also doesn't want to be out of the picture. He's still got the belt and very capable of challenging either Casimero or Inoue if camps bring him a good deal after this fight.