You can also cover him in Sulfuric acid and watch him scream in horror

You can also make a tub of Acetic acid, because said acid is weak make him drown in it, if he sucks it in it tastes bad so it is worse than just drowning in water

Btw sorry if anyone gets creeped out🙃 s' not my fault ya read this

Because hes bone it's simple to use a chainsaw in order to cut through it but because gaster can heal himself that won't work straight of the bat so first tie up the hands tightly that will slow the process down a bit next have a mechanism that hurts him somewhere else so it will only heal one spot than start the chainsaw and cut him limb from limb because it won't heal back, if you want to take a break patch the Brocken parts so they have the inability to heal