Assuming that you do not care the efficiency of power plant and transmission line (assuming 60% efficiency for combine cycle gas turbine) vs gas pipeline, then gas dryer will be more efficiency. If I remember right the gas water heating should be at least 60% efficient.
Gas will be cheaper for sure in terms of energy, but you might pay more for the dryer and repair / duct cleaning.
I think it is also more efficient to use the lowest heat level to dry your clothes, because natural air circulation will also take out moisture, in addition to heating up the air, so the additional air volume will help bringing out the moisture.
Drying 1 fuller load of laundry should also be more efficient than drying 2 lesser load of laundry, but the problem is if you do not dry it enough and have to dry it again, you might waste half of the load if it only need 1/2 a load to dry and you didn't stop it in time.
The key is to use just enough heat and just enough time on just enough load. If you over heat, dry too long, or have to dry twice instead of once, you will be wasting energy and time.