Do I need any patches to play the Gothik 2 Gold Edition from GoG?

I want to play plain vanilla also because the game shall run on a nearly authentic hardware (Pentium 4, Windows XP etc).

No, the GOG version is good to go. If I were you, I'd put off on that "original hardware" thing though, Gothic 2 was pretty demanding back then, especially on the GPU, with the render distance being pretty high for its time. Unless you run it on a pretty high-clocked Northwood-series and something like a Geforce 4 or 5, I don't think you'll have much fun.

The GOG version runs fine on modern systems, so there really is no need for this.

Thanks for your warning! But Gothic (1) worked fine on the hardware, so I hope Gothic 2 will do also. Am I wrong?

Even this Gothic 1/2 SystemPack is not necessary while playing Gotic II with Add-on under Windows XP?