Most likely the child would be 66% Gem with a singular personality. The system could occur with a fusion, but likely the fusion would have some rather skewed abilities. It is possible that they could recieve either fusion abilities or individual abilities based on fusion components.
You do mean gem+human children, don't you? Well, if so, that would make a real twist in the storyline.
First of all, the GAP team would end, changing the series like nothing has changed it since Mirror Gem/Ocean Gem... stop, cross that out, it'll change the series like nothing has changed it since the Pilot!!!!!
And, in addition, there would be a human character to sire that child. Only try to imagine a love bigger than the one that Ruby and Sapphire have to each other, a love that makes them both take such a big decision!!!!
To sum up, it would be a very, very, VERY radical event... but I need to see that Fanart RIGHT NOW!!!! GEM TWINS!!!!! OMG!!! ASDFADFASDFASDFASDF
If that would happen would she give birth to two children? Also who would be the father of the children? I guess those are fun things to think about, but I hope that doesn't happen because Garnet is one of my favorite characters on the show if they killed her off I would be devastated.