Hi can anyone tell me what gasket rubber is and how you use it.
I thought it was hobby rubber,but i'm not shore.

Please let me know!

If you are near a hardware store or Home Depot/Lowes you can find it in the plumbing section. They sell them in small sheets.

This is used for flexible or smaller mouthplates in puppets such as Kermit, Robin, and very early Bert...

and can i use vilt on it?

Yes, I use the self adhesive felt sheets...

And this kind of sheets?

http://www.joeppie.nl/knutselen/hobby-rubber.html

The ones I buy only come in a brick color, so I really can't be sure by the picture it those are the same or not...

The problem is i don't know what the word gasket means,so i don't know how to explain that in a craftshop.

Jeroen,
What you are calling hobby rubber, I think is what we call craft foam. It is not as durable as gasket rubber but you can use it for a mouth plate.

Gasket rubber is used both on plumbing and in cars.  I tried to find an example or source in the Netherlands but could not.  

In the translator, Gasket Rubber translates to Pakning gummi.  It is something kind of like this. http://www.dafa.dk/produkter/gummi-/og_pur-produkter/pakninger?opendocument  The rubber that some of those are made of can be purchased here in the US in sheets at the hardware or plumbing stores so that a person can cut out their own gaskets or pakning.

Basically Gasket Rubber is exactly what it sounds like. A gasket is the small rubber rings you will find in common hoses and pipes. Mechanics and plumbers buy flat sheets of actual rubber and cut replacment gaskets for pipes and hoses that may have dried up in time.

Try maybe eBay or Amazon?