If one could, it would be like other self-launching sailplanes (not TMG) and would need a licence. An ordinary PPL will not do – these are too different from normal GA types or TMG’s. I am not expert enough to say what will happen post EASA/April 2015t, but I expect somebody will explain.
I think this might actually be an area where EASA will simplify things a bit.
To fly a glider, you will need an Sailplane license (SPL) or Light Aircraft Pilot's Licence (Sailplanes) (LAPL(S)). They are pretty much the same, except for the medical requirements.
You also need to be trained in (and signed off by an instructor if you add it later) for the launch method - winch, aerotow, bungee or, (in the case being considered), Self Launch.
To fly TMGs, you need to add a TMG rating to the SPL or LAPL(S). You will also be able to add a TMG rating to a PPL or LAPL(A). So glider pilots will more easily be able to fly TMGs than they can now.
Paul