I very strongly doubt that it is a sin at all. If I may be so bold, I think the idea that it is mortal absurd.
To be considered gluttonous, an act of eating or drinking must be “excessive.” “Excessive” is not an absolute term – it is different from person to person and situation to situation – and we may perhaps define it as an “undue indulgence,” either consumed to the point of deliberately and knowingly injuring the physical or mental faculties (e.g. drunkenness or wanton and deliberate overeating to the point of obesity) or consuming exclusively for the pleasure of it (and deliberately excluding other goods, perhaps by forced vomiting or eating styrofoam or something).
If you’re eating more than is normally recommended for your age group because of a medical problem (chronic nausea), of course it can’t be gluttony! And if you’re doing it because of another medical or emotional problem (depression) the same holds true. It sounds like you’re not obese, you haven’t disabled yourself or seriously injured your health through eating, and you’re not going around thwarting the other goods of food (like nutrition), so… none of this is an “undue indulgence,” and you’re really in the clear here. I could be wrong, especially if you’re using food to avoid dealing with whatever’s making you depressed in a more responsible way, but, if it’s just a release valve for you that isn’t injuring you and isn’t completely self-indulgent, it seems to be really okay.
Quoting Catholic Encylcopedia here:
Gluttony is in general a venial sin in so far forth as it is an undue indulgence in a thing which is in itself neither good nor bad. Of course it is obvious that a different estimate would have to be given of one so wedded to the pleasures of the table as to absolutely and without qualification live merely to eat and drink, so minded as to be of the number of those, described by the Apostle St. Paul, “whose god is their belly” (Philippians 3:19). Such a one would be guilty of mortal sin.
I, for one, would be more worried about that scrupulosity.
Disclaimer: I am not a theologian. The advice of the poster is entirely his own. Always see a priest for more definite and personal guidance.