Since Greg and Rose took a video of the whole entire band thing, Greg watches Pearl and Rose fusing over and over again. He basically wants to dance like pearl.  Eventually, Garnet says to him that he has to make his own dance. 

Greg drags Rose to a stage and dances with her, the stage lights up and everything. Rose is so amazed but when she finds out that he was trying to fuse with her, she just laughs. "Its impossible!" she says.  Then Greg realizes that he barely knows Rose at all. 

Then they just have a conversation about how they barely know one another.

Eventually that all passes and Rose has to make a choice, a choice to give up her physical form to bring Steven, the baby into the world.  It's said multiple times throughout SU that "It's not easy to understand Rose's choices.."

Rose made the choice to, I guess you can say, "Convert into Steven."

To me, it makes sense that Greg and Rose are a fusion. Steven has the musical aspects of Greg and Steven also has the Gem stuff that is the aspects of Rose. Im guessing that if Greg wasn't a human, than he would have "Greg's Gem" but since Greg is human, like I said before. Rose isn't a human so Steven has Rose's Gem.