An Underfell Gaster Blaster is not very different from an Undertale Gaster Blaster. Although, in Underfell, the Gaster Blasters are considered to be more vicious and stronger than the alternative normal Gaster Blaster. It fires beams that rival the speed of light and has supposedly been "manifested" from W.D Gaster, it is seen being used by Sans frequently especially in the Genocide Route.
In terms of appearance, the Gaster Blaster truly has various intimidating features that would normally infatuate the player to great lengths. The Gaster Blaster has red eyes that grab the attention of the player very easily with an ominous smile that appears to stretch near the top of the eyes. There is an eerie fluid that is leaking from the eye which would probably give a feeling of dread to the player. Overall, there are not many major differences that can be spotted between the normal version and Underfell Gaster Blasters apart from the red eyes and shape of the smile in terms of dramatic effect being portrayed.
Note that this is speculation as there isn't enough information about Gaster's history to prove as a fact.
Obviously, the Gaster Blaster is a significance hence the name by the very famous scientist that was previous regarded as "Former Scientist of the Royal Guard".