It's just random wingdings.
In American Sign Language, they would be, from left to right: F(or 9) V 10(or APPROVAL-GESTURE) YOU(or GO) and DISSAPPROVAL-GESTURE
So far, we only know he was a scientist who had an accident in some undefined place. He probably knew Alphys and Sans, as they are both scientists who either utilize or make mention of his work such as the Gasterblaster, the thing that makes Sans so goddamn powerful .
Because of the incident, he is now a technical glitch in the universe, who does not exist, yet does.
I know close to nothing about Gaster, and he's already my favorite character of the game. At least he has some uniqueness to him. I can't ******* wait till Toby does more with him.