is it allowed under the GPLv2 License to modify anything on my wordpress installation and resell it?
Yes, as long as your modifications do not do something like encrypt or obfuscate the code and you do not put restrictions on the code including your own modifications. That encryption or obfuscation would restrict the freedom of the end user. That's a no-no for the GPL.
Also you must maintain the credit for that code as it wasn't yours to begin with.
So if you modify any GPL code and sell it you can't say to your customer "you may not distribute or modify what you have bought" or restrict the use and/or modification of that code in any way.
2) the source code must be published to the world (?). How can I do this? Does it mean the source code has to be put on git or any other known network?
You're in luck: with PHP it's not a compiled binary. ;) So any PHP that you distribute will be readable in any text editor and the user will have the source code already because you've given it to them.