IMO, it's not really a huge fight, although Gaballo is younger and undefeated the quality of opponent Donaire beat is higher than what Gaballo had beaten, it's a big test for Gaballo but for Donaire, it should be an ordinary day in the office comes the day of the fight.
While I agree that Donaire is without a doubt the best of the two and this must be reflected in the odds about who should win the fight, at the same time I do not think it is healthy for him to have the attitude that since he has beaten better fighters in the past then he's going to beat his current opponent with ease and that there is nothing to worry about, we have seen that attitude many times in the past and it never ends well, boxers need to take seriously their opponents because only one punch is needed to change the outcome of any fight.
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.
For sure they cant really afford to lose and losing their belt as it would really lost up interest in part of Inoue on whosever would really tend to challenge him up.It doesnt matter if its Donaire or Casimero
as long both fights could possibly make a match with Inoue but i would much prefer on seeing Casimero in that case but well its an another topic.
Gaballo is still a worthy opponent though even he do lacks experience but still he shouldnt really be that confident.