Violate the GPL - can I say it is mine?

Hi everybody,

I stumbled over this http://community.pcspezialist.de
the subsequent community of http://www.snippr.de/

As far I know snippr was an Elgg based community platform. And now a friend has sent me http://translate.google.com/translate?hl=de&sl=de&tl=en&u=http%3A%2F%2Fwww.hdnet.de%2Fneuigkeiten%2Fartikel%2Fwwwsnipprshopde-geht-in-die-zweite-runde%2F in which the developer talks about a custom-built community application. Is the new site http://community.pcspezialist.de not Elgg based anymore?

But at all this is not really the question. I just want to be sure. Can I use Elgg and say it is mine without telling it is Elgg and/or it is published under GPL2? Can I customize it and can keep it closed source?

Thanks
Gullo