I am not real familiar with Gargraves. I don't think the Gargraves flex track would be suitable for a temp layout, but the sectional probably would be.
I use Atlas track and before making a more permanent layout last year I used it for temp layouts for about 2-3 years. The rail joiners may loosen up after repeated use, but you can tighten them with pliers. May have to push the joints together once in a while as well, after repeated use, but it stays together fairly well. May not be quite as good as Fastrack for temp layouts, but quite adequate.
I had it on a make shift table with a bunch of plywood layers that were kind of tilted and quite uneven. On carpet or a flat level surface I think it would work much better, but have never tried it on either one. My temp layouts were pretty cobbled together, Rube Goldberg would have been pleased with them.