Thread:Project:Support desk/GNU GPL/reply (2)

2) If the MediaWiki's GPL covered your website (it doesn't), then advertising and other commercial use would be ok. The GPL does not prohibit commercial use of covered items.  It may limit some business models, though.

3) If you create a derivative of (say) the Vector skin, the GPL only covers your use if you decide to make your modifications available to others. The GPL requires that when you give other people your modified skin, you also give them the source code and the same rights you have under the GPL.  Since this is PHP, HTML, and JS, if you give someone the skin, they have the source.  The only real difference is that they can redistribute your work and you can't stop them.

It doesn't sound like you want to distribute your modifications, though, so this doesn't cover you.

However, IANAL. You should talk to a lawyer familiar with the GPL if you are really concerned about the owners of the copyright (Wikimedia et al) coming after you.