Training to wee in 1 area would be good in theory. I just need to work out how!!
First train your dog to wee on command - send them out for a wee and repeat your command work (I use a high pitched ‘peeeee!’)
When she starts to squat, give lots of praise (‘good peeeee’) and a treat.
Repeat every time you let her out. Most dogs will get it within a couple of weeks, some will get it a lot quicker
But it’s a really useful command to have for when you’re about to leave the dog at home, or when you stop for a pee break on a long car journey.
When you’ve reliably got her peeing to order, you can then introduce the place for her to pee. Gravel is good but some dogs don’t like the texture or noise, so you might need an alternative
Take her out on a lead or stay close to her, and lead her to the pee spot. Give the command to pee plus a name for the area (‘pee gravel?) and when she pees there, give lots of praise and a treat.
If you see her squatting to pee in other parts of the garden, just lead her to the gravel and give the command again.
Repeat every time she goes outside, adding the stress to ‘gravel’ and dropping the stress on ‘pee’
If you see her going to pee elsewhere, a quick ‘ah ah’ and then move her to the gravel
Then you’ll have 2 commands. One to pee on the gravel at home, and one to pee on command when you’re in the park or wherever else
After a couple of weeks of being made to always pee on the gravel, it should just become habit for her