What's gagiin and boubou mean?

Burn, baby, burn!

Gagiin

It's probably like shakiin which is just the sound effect for a flash sound. Either that or it's the sound of scraping metal sliding into place.

Boubou

This one's easier to answer since I can just grab it off a dictionary. It's the sound of something burning. http://dictionary.goo.ne.jp/leaf/je2/69893/m0u/%E3%83%9C%E3%83%BC%E3%83%...