GSPs have been spawned in labs but only very occasionally. If I recall correctly, they breed in seawater; or at least, the sperm are only fully motile in seawater, so fertilisation is less likely at lower salinities.
Actual reports of successful breeding by hobbyists are very rare and mostly anecdotal, without consistent details on things like mating behaviour or how to sex mature fish. Older aquarium books mention cichlid-like behaviours and nest-guarding by the male, but given the confusion over identifying Tetraodon species in the past, I'd take any such reports with a pinch of salt.
Cheers, Neale