"With 24% less weight and 76% more power, it should easily beat 600 fpm"
Don't forget the lower drag due to streamlined pylon compared to traditional installations like ASW24E.

The equation to determine climb rate is relatively simple, the main unknown is the thrust delivered by the prop at ~25-30 m/s climb IAS.

For the GP15, modeling a decent 1 meter 3 blades prop gives 550 N thrust at 30 m/s climb IAS with 25 kw on the shaft, sea level ISA.
AT 325 kg (722 lbs) TOM, L/D of 40 due to pylon, the climb rate should be ~4.3 m/s (~850 ft/mn).

Modeling the ASW24E with the same equation and following assumptions:
Thrust 450 N at 26 m/s IAS with 17 kw on the shaft (just a guess, using 20% higher prop efficiency than the GP due to larger prop diameter)
Weight 425 kg and L/D with pylon out 25
the climb rate is estimated at ~1.7 m/s (~340 ft/mn)

Even if these estimations are for sea level ISA, GP15 with such a high climb rate should perform very well even at high altitude in summer.
With the large battery packs and such a high climb rate, the GP15 should deliver an excellent climb altitude of ~4500 m with above assumptions and a 10% energy reserve. This is a very good differentiator!