I can not set parameters for Gauss Kruger 7 zone Serbia
I personally had no luck with predefined coordinate reference systems in QGIS or any other CAD/GIS software for the area of Croatia (Gauss Kruger 5 and 6 zone). If the coordinate system for QGIS that Oyvind proposed does not satisfy your needs, you can try this one. You just have to define it in Custom CRS dialog.
+proj=tmerc +pm=greenwich +lat_0=0 +lon_0=21 +k=0.9999 +x_0=7500000 +y_0=0 +ellps=bessel +towgs84=574.027,170.175,401.545,4.88786,-0.66524,-13.24673,0.99999311067 +units=m
Datum transformation parameters were taken from this site.
The coordinate system you are looking for is probably MGI/Balkans Zone 7. In QGis, it is located under "Projected Coordinate Systems" -> "Transverse Mercator" -> "MGI/Balkans zone 7" (EPSG:31277) in the CRS selection dialog.
In case of a layer, right click the layer and "Set layer CRS".
If you want to set the project CRS, there are two options:
You could use this 3 parameter CRS definition in proj4 format:
Parameters are taken from AGROS - Active Geodetic Reference Network of Serbia. They are semi-official but they work pretty well.
If you have some permanent point with known Gauss-Kruger/WGS84 coordinates you could check it.
Example> Point 15491 (data source:Geodetic Network of Serbia - http://www.geomreze.rgz.gov.rs/