Regarding gastroparesis: As CalGal said, there's no "cure" for it because it's not understood all that well. While it's often mentioned as a condition, it's been my observations that most times it's actually a symptom of something else. Depending on what that "something else" is, the condition may be "curable" (or rather, treatable enough that it might as well be a cure.)
If you've spotted a pattern with the menstrual cycles, go ahead and have the hormone levels checked. In aryuvedic medicine there is an herb, shatavari (a type of asparagus) which is used for both stomach issues (dyspepsia), and for female hormonal issues. (It's usually refered to as a woman's herb.) The herb has been shown to work in a very similar fashion to the drug Domperidone, which is a drug used in both gastroparesis treatment and in the treatment of women in regards to lactation. (My knowledge in that area is lacking, so you'll pardon me if I don't go deeper into the way those work). Additionally, traditional chinese medicine recognizes ties between weakness of the stomach and a lack of "Yin" (female) energy within the body. So you're not alone in believing the two may be tied.
You may do well to find a good, reputable holistic physician in the area in addition to your traditional doctors, to see whether the condition can be improved via those paths.
Good luck.
Unfortunately, there is no cure for gastroparesis. Except in the case when the condition is caused by diabetes, no one really understands why it happens. Hopefully, you daughter is following a gastroparesis diet and that will be of help.
There are medications that can be tried, and if her case is severe you may want to consider a gastric pacemaker.
At this point in time hormones are not supposedly implicated in gastroparesis. If you feel there is a tie-in with progesterone, why not consider having her hormone levels tested and if they come back skewed, try to modify that state.