The few youtubers who are the most professional about it (I.e. : like the two I posted) don't do it inside in a climate controlled building that's set up so that the gel doesn't turn into a big blob within a couple hours if that's what you're asking. Most ammunition companies aren't even set up for that.
(As an example here's a
Test at Federal (*Click for link*) - See? It's not in a building set at the same temp as the block [39 degrees Fahrenheit])
Gel test at Federal Premium in Minnesota
It goes directly from the freezer into ice chests and then is carried outside a short distance to their yard where they have the cameras set up and then they pop it.
It's not stuck in a car for a long trip. Nothing involves a gravel pit (although that would be fine just so long as the molds and freezer was in a trailer at said gravel pit and they were able to get the block at 39 degrees Fahrenheit and kept the time constraints in mind - that the block cools 0.6°F
every four minutes if in an outside temp of 80 degrees F).
I'm not sure why a gravel pit keeps coming up. Did you see a video involving some complete doofus with milk jugs and wet phone books at a gravel pit or something?