Mucuna Pruriens contains highly active amounts of L-DOPA [levodopa], a direct amino-acid derivative of L-Tyrosine, and the body's direct precursor to Dopamine synthesis. This is important to the discussion as L-DOPA displays a marked ability to increase Testosterone and LH in healthy, normal males, through the inhibition of PRL [prolactin]; it does so by increasing Dopamine production in the hypothalamus. In turn, Dopamine site-binds to D2 [dopamine] receptors found on the anterior pituitary cells responsible for PRL secretion, "lactotropes". This direct site-binding by Dopamine alters Ca2+ currents in the lactotropes, and also promotes a reorganization of protein lines responsible for cell structure. In combination, this inhibits PRL secretion and promotes healthy LH, and Testosterone synthesis.
Now, I say this because PRL is also known to increase the expression of ER-A [estrogen receptor alpha], the estrogen receptor sub-type responsible for the proliferation of breast-cancer cells. The selective modulation of this receptor is the target of certain drugs in the drug-class SERMs [selective estrogen receptor modulator]. The inhibition of PRL, therefore, has been demonstrated to decrease ER-A sensitivity to estradiol [estrogen] and decrease ER-A related gene expression. In concert, these inhibitory effects on PRL would most likely attenuate breast-glandular expansion [i.e., development of breast tissue associated with gynecomastia].
In simpler terms, Mucuna is highly unlikely to promote gynecomastia, and it all likelihood works to decrease factors that promote it.