Remember that G-lines have nothing to do with mIRC - they are solely set by the network's IRC Operators, and they can set it on whatever they like, including entire ISPs. See
this thread for information.
The best thing to do is wait 24 hours and then try to reconnect. If it still has not been removed, go to
www.undernet.org and see if you can find contact information for the people who deal with G-lines. Make sure you include your IP/hostname/ISP name, and the entire G-line message.
G'luck!
Regards,