You probably know more about them than I do tetra (jason)

but I have read that the males will often dig and tunnel through gravel, particularly when they mature and get to breeding age..they build a tunnel near a cave or rock etc.
He may also feel a bit threatened at the moment due the new gouramis presence, plus if his usual dark hiding spot during the day has been recently disturbed by another fish, he may just have choosen to hide in the gravel.
I wouldn't be concerned!
