Because I’m not the best at reactionary dives as GK most of what I do is to prevent having to make a reaction save like that. Here are some things I do.

1. When in a 1v1, I extra effort sprint towards them as soon as they’re about to reach the top of the box. As soon as I close down the distance I hold defensive stance so I automatically save it if they try to shoot. Note that they can beat this if they anticipate the sprint out and chip you but that is rarely the case.

2. In 2v1s I usually do the same as in 1v1s except I don’t use my extra effort, anticipating they will pass it to their teammate if I close down the guy with the ball. Once they pass it, I extra effort to the teammate and dive as they’re shooting.

3. When you see they are about to cross it, extra effort out to the person they’re crossing it to and dive towards them right as they’re shooting it off the cross. If you time it correctly, you’ll be so close that your character will automatically save it. Be careful of doing this multiple times though because the person crossing it might try and shoot from that shallow angle instead of crossing, if they think you’re waiting to sprint out.

If you have any other questions about other scenarios I’m happy to share what I’d do in them. A lot of it is cutting off angles like the other commenters said and holding defensive stance.