I have a very strict/ structured care routine when it comes to my
wetsuit boots.
I don't tend to get them wet before putting them on as it's not really that awkward, plus then you don't have to get your feet all cold before getting in the sea.
My routine is this:
I trudge wearily up the beach through sand, gravel, tar, broken glass etc.
I walk across the tarmac of the car park
I rip and tear them off in any way that lets me get out of them quickly, empty the water out and sling them in a carrier bag. I then usually leave them until the next time I go surfing or if it's going to be more than a couple of months I will sling the carrier bag in the garage and forget about them. One day I will open the garage door and be greeted with the foulest stench imaginable, so I will open the bag (whilst holding my breath) chuck them in a bucket of water, slosh them about a bit, then sling them back in the garage, where I will spend hours trying to find them both next time I need them.
I find this works very efficiently and I have had my boots for 10 years!!
In short, I wouldn't worry too much about the gravel thing and just rinse them out when you get home. Job Done!!