GdkGC with the given depth, colormap and GdkGCValues.
If a GdkGC with the given properties already exists then it is returned,
otherwise a new GdkGC is created.
The returned GdkGC should be released with gtkGcRelease when it is no
longer needed.gtkGcGet(depth, colormap, values)depthGdkGC to create.colormapGdkColormap (FIXME: I don't know why this is needed).valuesGdkGCValues struct containing settings for the GdkGC.