The goal is to put some 3D objects on top of the camera.
I have created a Camera View and another smaller GLSurfaceView (wich renders the basic 3D Cube) on top of the camera preview.

I can see the 3d Cube spining correctly however it is framed inside its GLSurface, wich is black. I want these frame to be transparent in order to see the cubes as if they were spining on top of the camera preview.

Is this possible? No idea of OpenGL.

Yeah this is possible. You basically have to paint the OpenGL scene using a transparent background. You can check out an example I wrote that is exactly doing that.