You just summed up the entire movie plot in less than 10 words, that's my point lol. There is absolutly no build up of characters, the movie starts with them in space already, there's no build up of who they are, where they really came from, people in their lives, things they did before they were at the point of being trapped in space. The whole thing about Sandra's daughter dying almost like a writers afterthought and was just a weak attempt at a quick backstory that they summed up in less than 30 seconds of the movie. So there's no connection for the audience to actualluy care one way or another about the main characters. The whole plot is she's trapped in space and has to get back, IMO it's lacking in plot because there's nothing else to go along with it.
I'll give you an example using another space movie, Armagedon. In Armageddon the first part of the movie was building the characters by showing their backstory of who they are, why they are there, the people in their lives, the relations between Liv Tyler and Ben Affleck, how and who would be affected if they died, the relationship betwen Bruce Willis and Ben, even the smaller parts like Steve Buscemi's character and Micheal Duncan had character build ups which added to the plot which in turn makes you as the viewer feel a connection to the character, which also in turn makes you care about what happens to them in the movie. In Gravity....it's two people in space with no story other than "oh we're trapped and have to get back".....And? lol. There's just nothing to make you care about them. Plot just seemed really really thin to me man, almost like it took no imagination to come up with. I felt they tried to rely way too much on CGI and really put no actual thought into the script or plot if you ask me.