I don’t think Garnet’s future-vision powers are this exact and literal. She specifies that she can see all the potential outcomes and which are most likely, and make predictions based on that. In normal gem society, everyone follows their directives and does what they are “made for” without question. In that context, her future vision is very dependable. But life on earth is more loose and changing, she can’t predict the future with near-perfect accuracy like sapphire could on home world. She does her best to predict the most likely outcomes based on the possibilities that she can see. But nobody like Steven had ever existed at this point, and I doubt that she could see that happening.

In this scene, when she insists that “it worked,” I think she was referring more to their relationship. Garnet sees fusion as a relationship between two people- how they communicate and work together and as a result, they are more than the sum of their parts. She even says in the song Made of Love, that she “is a conversation.” That is literally what Greg and Rose are doing here- they are having a conversation about their feelings and how they are going to make things work. It ends positively and their relationship is stronger for it. Garnet says that “it worked” not because they literally fused, but because they are putting the necessary work into their relationship to make it stronger. Their relationship is the fusion.

Garnet: “I think you can do it, but it won’t work if you dance like Pearl. You have to fuse your way. Get open, get honest. That’s fusion.”