1. First create any model in BRL-CAD.
2. Run command prompt in the directory where you saved your .g model.
3. Convert .g model to .obj model by typing following command in your command prompt:
   g-obj -o model_name.obj model_name.g object
4. Open FreeCAD, create new project, click File->Import, choose your model, small window will appear. Select Alias Mesh (Mesh).