From what I've heard the issue is that their clutch sizes are too small to be commercially viable. Otherwise they should be easy enough to breed.
Mine are most definitely spawning, but I haven't been able to find their nest - way too many rocks in that tank to hide in.
Grammas I had in the past build outright nests from algae filter nylon and what other stringy material they could find. So I never could find any larva in the tank.
If I find the time I may look into this again, but only as training in case I can get my hands on a few G. dejongi.