Buried conduit is always considered a wet location. But if the conduit is simply to fish gas tubing through, it's probably legit, though I'd be concerned with galvanic reaction if the conduit is anything but plastic or the same material as the gas line.
If he has used the conduit intended for power, then you'll need another conduit.
One always defers to AHJ, but around here, soft copper is OK for underground hookup with LPG.
Personally, I would never put steel in the ground, even if allowed.

No only is it legal, code requires that when running any line under a concrete slab the line is required to be sleeved.

To make the required sleeve you first install a pipe that is at least two nominal trade sizes larger than the desired pipe, then run the desired pipe through the sleeve.

Well, I called the source, my propane co.

1. They can run flex plastic gas line inside a PVC sleeve.
2. The sleeve needs to be daylighted at each end so that they can attach steel sweeps to the plastic gas line. (There must be a certain depth of steel line below grade).
3. There then must be pvc sleeving slid down over the steel line where it might otherwise contact concrete.

I'm going to dig out each end of the conduit and cut off the sweeps the contractor installed. The propane co will then be ready to go on Monday.