Is there a gentle way to treat retained placenta?

Probably not. The term "retained placenta" implies that the placenta has failed to detach spontaneously with the usual postpartum measures including uterine massage, IV pitocin, (oxytocin) and gentle umbilical cord traction. At that point manual removal of the placenta is indicated, and while performed carefully may not seem to be gentle. If manual removal is incomplete, surgical removal by curettage is usually next.
DEPENDS. It depends on how long the placenta remains attatched to the uterus and the cause. Most retained plcenta come out by just waiting. Sometimes using medications(pitocin (oxytocin) or cytotc) help if that does not work and bleeding increases, it might be necessary to place a hand into the uteus and manually remove it. Rarely surgical intervention is needed to remove the placenta.