Gaster usually wears a cloak, resembling the River Person's cloak. But when he's not wearing his cloak, he is usually wearing a black mini-cloak. His hands can float around his body, resembling weird symbols, mostly in the WingDing font.
Before the events of AnomolyTale, he was in the void, unremembered. But when Papyrus died, people started to remember him. Finally, Sans tried to let him free, and it worked. Since then, he has been the Royal Scientist assigned by Asgore. His assistant is Alphys, but sometimes he works with Sans.
Gaster loves to experiment with different things. He experiments with both monster and human souls, resulting in the Amalgamates. He makes different foods, like baked goods.