Gaster Blasters are weapons used by Sans, Gaster, and Papyrus. They appear as skulls that release blasts of energy out of their mouths.
Attacking and Blocking: The Gaster Blasters are mainly used to attack enemies by releasing a blast of energy. It can be used as a shield to block attacks, and can even block other blasts from Gaster Blasters, its lower jaw is usually gone when it's blocking.
Giant Gaster Blaster: There is a much bigger variant of this weapon used by Gaster when he has 50% of his power as shown in Do or Die. This Gaster Blaster is much bigger and uses a more powerful blast of energy. Gaster can also use this when using Polychromatism.
Gaster Blastermination: Sans has a special attack in which he summons a big wall of Gaster Blasters towards his enemy. This attack is powerful enough to break through shields with little to no effort. This attack, however, drains much of Sans' energy.
Travel: As shown by Papyrus in Hate, Gaster Blasters can be used to travel at fast speeds.
The form of the Gaster Blasters reflect the users eyes, and even facial features in some cases. For example:
Gaster's Gaster Blasters have red and blue irises, when Sans died, it was just one red. When he gained all his power back temporarily in Animosity, his eyes were purple, and both eyes of the blaster were (except for one instance when they were red). His blasters also have cracks to reflect the cracks in his face.
Sans has one blue eye, and so does his blaster. His blasters also are drawn having a smile, and Sans is known for constantly smiling.
Papyrus has one red eye, and so does his blaster.
Papyrus uses his Gaster Blasters for mostly defense but will use it for an attack on something that can’t feel remorse.
Gaster Blasters are exclusive to monsters only.
When Sans was under Bete Noire's control, his Gaster Blasters got affected as well, which can be clearly seen, due to them having pink/blue eyes and pink colored blasts coming out of them, instead of blue.
It seems Gaster Blaster get their eye color depending on the user (presumably), Sans's blasters have cyan eyes, Papyrus's blasters have red eyed, and Gaster's blasters can either have one red eye, both eyes purple, and shown in Camilla's Camiart, it can have a red eye and cyan eye. When Bete Noire controls them with Rhabdophobia, their eye colors are pink, or have a pink tint on its body/head.