The G rating basically means any and every one can watch, without having to worry about the content or anything that could be distressful or traumatic for younger audiences (though, that isn't always the case, entirely, if older Disney movies, or even Don Bluth movies are any indication of that).
The problem is, as we've discussed on this very forum many times before in the past, the G rating has become stigmatized over the years: it's become a dread for cinema, because a movie with a G rating is automatically a "kiddy" movie, and parents don't want to drag their kicking and screaming kids to a movie that they don't want to have to sit through themselves. More often than not now, movies that should be rated G without question now opt for a PG rating, because it infers that there's a little bit of somewhat edgy, risque, or tongue-in-cheek content for the parents to enjoy, while also making the kids feel like they're mature enough to see a more "grown-up" movie with their parents - even if it's just a mild swear word, or a subtle joke that goes over the kids heads.
In other words, a G rating isn't marketable anymore because of the negative stigma it's earned.