This is very good and I really love Gravity Falls and it is so hard to find good GF artwork on NG. There is one issue though I believe. I believe her skirt is rather shorter than it should be, if you are going for the original outfit from the show. Remember that the proportions in the cartoon are off from the actual proportions of a more realistically drawn person and maximize the usage of the face and hands to allow the viewer to focus on the character's overall expressiveness. In your case, since you altered the proportions, the skirt would end up larger because the legs have become more defines and much larger proportional to the rest of the character, when compared to the original art style.

Otherwise, it is perfect.