You'll need to reproject them to handle this conversion.
Proj.4 is a very standard library, used by a lot of software for reprojections.
One means of using this would be to use a utility like gdaltransform to do the transformation for you.
I see you are from Sarajevo so you are probably need to convert GK coordinates from B&H coordinate system. First you will need to get transformation parameters for your area. I don't think there are any good one in EPSG database. Maybe you can use this one. I sometimes use them for area of Croatia.
Regarding the C# lib, there is one which I used Proj4Net.
Here is one my old example.
CoordinateSystemFactory cFac = new CoordinateSystemFactory();
Wgs84ConversionInfo par = new Wgs84ConversionInfo(514.0188, 155.448, 507.0461, 5.6136, 3.676, -11.4667, 2.091);
IEllipsoid ellipsoid = cFac.CreateEllipsoid("Bessel 1840", 6377397.155, 6356078.963, LinearUnit.Metre);
IHorizontalDatum Bessel1840 = cFac.CreateHorizontalDatum("Bessel 1840", DatumType.HD_Classic, ellipsoid, par);
IGeographicCoordinateSystem gcsBessel = cFac.CreateGeographicCoordinateSystem("Bessel 1840", AngularUnit.Degrees, Bessel1840, PrimeMeridian.Greenwich,
new AxisInfo("East", AxisOrientationEnum.East), new AxisInfo("North", AxisOrientationEnum.North));
System.Collections.Generic.List<ProjectionParameter> parameters6 = new System.Collections.Generic.List<ProjectionParameter>(5);
parameters6.Add(new ProjectionParameter("latitude_of_origin", 0));
parameters6.Add(new ProjectionParameter("central_meridian", 18));
parameters6.Add(new ProjectionParameter("scale_factor", 0.9999));
parameters6.Add(new ProjectionParameter("false_easting", 6500000));
parameters6.Add(new ProjectionParameter("false_northing", 0));
IProjection projection6 = cFac.CreateProjection("Transverse Mercator", "Transverse_Mercator", parameters6);
IProjectedCoordinateSystem zona6Bessel = cFac.CreateProjectedCoordinateSystem("6 zona Bessel", gcsBessel, projection6, LinearUnit.Metre, new AxisInfo("East", AxisOrientationEnum.East), new AxisInfo("North", AxisOrientationEnum.North));
IGeographicCoordinateSystem wgs84 = GeographicCoordinateSystem.WGS84;
CoordinateTransformationFactory ctFac = new CoordinateTransformationFactory();
ICoordinateTransformation zona6ToWGS84 = ctFac.CreateFromCoordinateSystems(zona6Bessel, wgs84);
toPoint = zona6ToWGS84.MathTransform.Transform(fromPoint);
I hope this helps a little. This code was written a long time ago but i think it works. You just have to insert your parameters.
Best Answer
Your coordinates are likely in UTM (Universal Transverse Mercator) Eastings and Northings.
You can convert UTM coordinates to lat/long online using many different sites. Here in one. You can also perform this calculation in excel or inside of a database if you can work out the formula. Alternatively you can perform the conversion inside of a GIS such as QGIS or ArcGIS.
Before you can convert coordinates to lat/long you will need to know which UTM zone your coordinates fall in.
Here is an example for North America:
In this example, most of Florida falls within the UTM zone 17N.
I came across this page by searching 'batch convert UTM to lat/long'.