My GH sits on a wood deck surfaced with 2x4s. I lined the floor with polypropylene boat carpet. It's been there four or five years. Once a year I gut the GH, vacuum the carpet, and shampoo it with laundry detergent. By now the carpet is kind of stained up, but structurally still like new. The carpet is water permeable. The water just goes through the carpet and the cracks between the 2x4s, and the carpet dries in a fairly short time. I've never felt any cold air coming up through it in winter.

The idea with stones and gravel is that the sun heats up the mass during the day, and then that heat is radiated upward during the night. But if you're using electric heat, that's not as important, and heat sinks like stone can actually make your heater run longer under the right circumstances. A heat sink is also a cold sink.

If you put down a floor material, make it something comfortable to stand on, and safe enough so that you don't trip over it. Pea gravel can roll under your foot like a marble when it is on a hard surface.