Should Gaster allow himself to be tainted by the Hunger, the multi-verse would collapse. He would also look like this. Property of Mercy-monster.

As a denizen of the Void, Gaster is absolutely disconnected in every way from the Underground. As a result, he is completely untouched by the madness of the Hunger. His sons are as healthy and happy as they are because of his efforts in keeping them alive.

He couldn’t reach Frisk to save her, which is a flaw that he blames himself for every day. He had no power on the surface world to spare Aliza her lonely childhood or her struggles, but he helps all he can when Aliza reaches the Underground, as a creature unable to directly affect living beings.

He scares away the worst monsters. He keeps Flowey at bay as long as he can. He nudges her in the right direction, and whispers encouragement in her lowest times.