I know this question was partially answered already,but I would like to ask again as this will be new construction.As I plan my main drain under slab at 1/4" per foot is only one vent stack allowed coming off the drain or could there be another?Also I would like to know if pvc 40 and pex tubing is acceptable covered with gravel and then cement.Is the vent stack allowed to go out a gable wall and not through the roof?

No on the gable and yes you can have more than one vent if needed. Why are you needing to put PEX under the floor?

It's a panelized house with all exposed beams and we are trying to eliminate as many box ins as possible for the plumbing.

Yes with burying both the sch 40 and PEX after covering with gravel.