There is something completely off about invoking the GPL-2 here...
It certainly cannot be extended to the sprites and files that the server reads! By that logic, you'd say that every text/office document you create/open in OpenOffice should be made available to other people as well!!
Additionally, the GPL-2 has a design "flaw" which is easily exploited by server software: the modifications you make only need to be shared when you distribute the binary. That means you can't take a project like OpenOffice, improve it, and then sell it closed-source without also making the changes/code available. However, this also means you can take a server project like Apache, modify it, and run your server off the modified version without having to share said changes.
You must realize that this, when applied to eAthena, means you can modify the server to your heart's content and not have to share the source-code of the changes as long as you don't distribute the server to other people. Which really is no news since many servers have been doing it for quite some time now :[
Anyway, just wanted to clear that up.