There are hundreds/thousands of scripts out there. 'Leeching' is not generally a good thing with a chat program (which mIRC is).
G-lines are bans from IRC networks. They have nothing to do with mIRC. See
this thread.
Cyber channels are usually related to the act of 'cybering' which means having sex through typing to each other.
There are no laws that govern what you can and cannot have in a popup. Unsolicited advertising (or 'spamming') is a bad thing though, and something we won't help with. Just think, would what you want to do be annoying to others? (Flooding, repeating, lots of colours, spamming, etc.)
Please use the Search feature before posting, it will answer a lot of your questions - if you're new, your questions are very likely to be simple ones that have been answered before. Expand to 'All Forums' and 'All Posts' for best results.
Hope that helps.
Regards,