Both of them looks tough that is to say the fight is gonna be tough.
From the history between Donaire & Gaballo, indeed both of them are the best boxers they have ever won, it can be said that their boxing this time is (headline), although Donaire is one of the most talented fighters on his resume but, Gaballo is not to be taken lightly by Donaire, Gaballo is also a talented resumer along with his fighters.
I agree with what you are saying, boxing Donaire vs. Gaballo is more of a challenge, roughly leading to defending each other's WBC titles.
If Donaire is still on his prime then I can say he undoubtedly better than Gaballo, but it seems like Donaire is nearing his retirement while Gaballo is still in the process of climbing the peak of his career.
This could indeed challenge Donaire as he is fighting a rising fellow Filipino boxer. This fight will determine whether or not Gaballo will be stepping on the bigger ring. On the other side, Donaire has nothing more to prove, he has become a champion several times in four different divisions.
So, It's up to Donaire whether or not he'll take Gaballo lightly.
Neverthless, this could be another interesting fight to watch.