Only the black soldier fly larva. "Phoenix Larva" presumably sounds more awesome than soldier-fly-squidgy-maggot-thing and you can sell more that way. Many of the feeder 'worms' in the pet industry aren't worms, but various larva.
If you can get your gray to eat bona fide earthworms, that would be great. They're a great food source but some frogs will be adamant about spitting them out.
It can take a couple years for a gray to reach adult size.