-It would have to be someone on the Gnutella network who called them, right?
I mean, who else could it be?

It's not that hard to do though, since your current IP address is displayed on Gnotella. All a guy has to do then, is to enter your adress into an IP address search engine, and send a complaint to the ISP which pops up. They will then(maybe) contact you.

I think it's weird that they called you though, 'cause I'm quite sure that if I really genuinely needed to report an IP at some point, his/her ISP would probably do nothing....