Deca is not aromatized to progesterone. (Progesterone lacks the phenolic structure in the A ring, so is not the product of aromatization.) Deca is however aromatized to estrogen (1) (2), .
Deca binds to the progesterone receptor (3), but whether progesterone has a direct role in gynecomastia is unknown. It is thought that it acts indirectly by stimulating prolactin secretion. How this occurs is not known. Since prolactin is thought to be the ultimate culprit, prolactin inhibitors such as bromocriptine will prevent and/or reverse prolactin induced gynecomastia.
There are no studies confirming winstrol prevents gyno, although it is possible since in certain tissues it binds to the progesterone receptor as an antagonist. It does not do this in all tissues, and there are no studies showing it binds in breast tissue.
Im am not aware of any studies that show RU-486 (mifepristone) prevents or treats gyno. In fact, in patients that have been treated with mifepristone for Cushing's disease, gynecomastia has been reported as a side effect (4)
(1) J Steroid Biochem 1989 Nov;33(5):949-54
Androgen and 19-norandrogen aromatization by equine and human placental microsomes.
Dintinger T, Gaillard JL, Moslemi S, Zwain I, Silberzahn P.
(2) 1: Nippon Naibunpi Gakkai Zasshi 1986 Jan 20;62(1):18-25
[Aromatization of androstenedione and 19-nortestosterone in human placenta, liver and adipose tissues]
Yoshiji S, Yamamoto T, Okada H.
(3) Methods Find Exp Clin Pharmacol 1997 May;19(4):215-22
Estrogenic and progestagenic activities of physiologic and synthetic androgens, as measured by in vitro bioassays.
Markiewicz L, Gurpide E.
(4)
http://www.endocrinology.med.ucla.edu/cushing's_syndrome.htm