we did the same with our pup. Now we open the door and he goes right to the gravel and goes potty. It was about a two week process for us but he got it. He still thinks about going on the grass but before he lets himself he will run to the gravel. He's a smart little guy! It take lots and lots of patience, and you have to be willing to take the time to do it. Don't give up just keep at it!The way I trained my old dog to use a certain spot for the bathroom is I kept him on leash & walked him to the spot I wanted him to use & said "Go Potty". I did this every time he had to go. We stayed in that spot until he went, & when he did he got lots of loving- I made a big deal out of it. I didn't let him have the run of the yard right away as a puppy. This also helped put him on a schedule since he wasn't running around going to the bathroom without me knowing it. If I saw him going or about to go I picked him up (or walked him) to his bathroom spot & used the code words again "Go Potty". It takes time & it can get tiresome having to go out with him every time, but consistency is what works. I plan on doing this with my new puppy also.
That can be done outside of the yard by taking the dog to different places. That should be done anyway. The dog needs to experience walking on different textures & hearing different sounds. When my dog was at training class there was a specific spot outside the building that was designated for dogs to go to the bathroom. He learned to always use that spot when we were there, & it was a different texture than the spot at my house. But if you want the dog to only use a certain spot in your yard, you have to consistently train it to go in that spot.Just a word of caution- you need to teach them that it's okay to potty on different surfaces, and not just the gravel.