What exactly is the Poya-gun? In every episode so far, Ryoka shoots/zaps Yuta with her Poya-gun, to which Yuta usually exclaims "POYA-GUN! NOOOOOOOOOOOOOOOO!"

What exactly is the Poya-gun? It seems too advanced for her to have made it, but everyone writes it off as being a totally normal thing when she zaps Yuta with it. At first, I thought it might be occult-related, but seeing as how Yuta does not believe in the occult, it wouldn't make sense for one of his closest friends to use an occultic object in front of him on a daily basis. 

Also, I have no reason to suspect that Yuta is not human, so his recovery is a bit superhuman if the Poya-gun is like a taser.
It's an electricity based self defense stun gun, comedically shaped like a retro style ray gun, likely of a not very high amperage as it doesn't seem to have much of a disabling factor on Gamon. People probably don't react because it's anime and nobody really cares that the 'main' is being abused by his friend. Of course, stun guns (tasers) don't actually work like the show implies the 'Poya Gun' does (as tasers don't have ranged ability, relying on you touching the arc to the victim for it to actually work.)