Does your horse always rush or just in gymnastics? Has he does gymnastics before or is this his first set? Some horses tend to get a little anxious or quick to them the first couple of times. And then they realize (sometimes with a little help) that it's easier to slow down and take the jumps carefully. Which is why gymnastics are so great on a horse that rushes.
How adjustable is your horse on the flat? I just took on a new project that ran at fences and just took off on the backside of them. There was no slowing him down! He also had no 'woah' on the flat either. Teaching him to "woah" and "go", and lengthen and shorten have been amazing for him. If a horse can't woah on the flat, he will not woah over fences.
How is your pace in your corner heading to the grid? This is a biggie. This is an issue I think A LOT of people have and don't realize. One of the best pieces of advice i've heard was something I overheard in a convo between a grand prix rider and her student (ok, so I was eavesdropping, it's like free training advice!). She said, "get your work done in your corner". Every horse has a speed that is the most comfortable pace for him to jump. The trick is finding that optimum speed. Some are faster then others. I find that some people think slowing their fast horse down in the corner will help them stay slow through the grid/line. What's actually happening is that their horse can't jump well going that slow and speed up out of anxiety because it's hard for them to get over the fences well. With those horses it's important to get them forward in the corner and leave htem alone in front of the fence. I have no idea if that's what's happening here but just a thought. This is another huge issue with my new project.
People have suggested some good things. Try putting down some ground rails. Try some other combos of grids. Here are 2 of my favs.
If you have enough poles I think bounces are a fantastic teaching tool for horses. I love 3 bounces, 2 strides, 2 bounces:
It can be jumped both ways.
Or for teaching a horse to canter a fence, trot rails to a bounce, one stride, to a single.
(3 trot rails) (xbar)(xbar)(one stride)(vertical or oxer)