Gaster was one of those to survive the Toxic by wearing on a breathing mask. He found the dust of the kingdom's prince and did a experiment to bring the prince back so then one day he'll be able to rule the kingdom and bring it back to it's glory. He brought Asriel back with the dust remains and giving him the corpse of a flower. (This is actually an Undertale theory [except for the Toxic part] so why not make it Canon here?)

(That's how he looks like. The black is Chara's black tears and the dirtyness on his petals, leaves, and stem are the dust of his opponents. The red in his eyes shows that he's mad, and he has a cut on his face which is bleeding nectar ;-;)