Do I need joist hangers for deck frame on gravel pad? What is the nail size?
You don't need joist hangers.  Joist hangers are for use in spots where you're unable to put the nails directly through the frame into the joist already.

I built a shed with much the same construction, only my base frame sits on 2 6xx treated railroad ties embedded in the gravel bed, and I haven't had any problems.