You will probably get more than the ASW24E.
The very high drag from the pylon with engine, like the ASW24E, reduces the glider L/D below 20, which start to have a significant effect on climb performance. This is probably a 100-150 ft/min difference.
It is more difficult to get the published power rating from 2 strokes carburated engines than it is from electrical motors.

Concerning GP15, its pylon is very low drag and most important, it is very light.
For climb performance it is not the wing loading but the weight which drives climb performance. To takeoff at 8.5 lb/ft2 with the ASW24E you need to be at 425 kg. With the GP15, you only need to 330 kg for the same wing loading.
Because of the available shaft power, drag and weight differences, I have no doubt the GP will have at least a 30% climb advantage with similar takeoff conditions.