Is het een kopie of een crack van Internet Download Manager?

No. GDM is an independent program written from scratch in Rust, with no connection to Tonec Inc., the makers of Internet Download Manager (IDM). It contains none of IDM's code or artwork. Its windows are laid out the way IDM users expect, so you can switch without learning anything new — and because GDM is free, there is nothing to crack. Please don't download "cracked" download managers: they are a common way to spread malware.