Is it possible to overwrite 1 dll in the GAC of a live server of my customer? Otherwise I need to install the whole feature. In this feature are very much webparts and other stuff.
you could try gacutil
and the command: gacutil /i yourdll.dll
you will find it in the visual studio command prompt.
If you have a farm with multiple servers, you'll need to update your dll's in all of the servers. but if you have only one server, you can drag your dll to the gac folder or use the GacUtil.
In any case I would suggest you to deploy your dll using a WSP solution which will copy your dll to all of the servers on your farm.