@Conspiracynut:
I've come to the assumption that it's a term meaning seastone bullets, or simply a strange mistranslation.
Now, if they were going to use sea stone bullets, the manga would have used the name "seastone bullets" instead of using some code. What's the point in hiding it, especially when those bullets didn't do anything?
Plus, the actions afterward don't communicate that they're seastone. Otherwise they would have injured Croc instead of going right through him. That should be the proof right there. And no, a logia user can't manipulate his body so that objects can pass through them. No other logia user has shown that ability, so we should assume they don't have it.
So yeah, Gaol bullets are just some kind of special bullet used by Impel Down. What they are is unimportant since they were useless in the chapter they were introduced in.