Will these work on gravel or dirt as well as asphalt?

Yes. The wide footprint spreads load across soft ground so your jack feet don't sink or tilt. That's the whole point — they work wherever your rig parks.

Do I need hardware to attach these to my jack?

No fasteners required. These pads sit between the jack feet and ground — place them, set your jack on top, and you're done.

Can I stack two pads if I need more height?

Yes, they're designed to stack. Build the height you need, then set your jack on the stack. Just make sure the bottom pad stays stable underneath.