It was time for a tune-up. On Sept. 5, Gwen Stefani, Gavin Rossdale and their lawyers held an all-day meeting “to hash out their differences regarding parenting decisions,” a source tells Us. While the Bush frontman, 52, and the “Used to Love You” singer, 48, have been mostly amicable coparents to sons Kingston, 12, Zuma, 10, and Apollo, 4, since their 2015 split, “the main issues involved scheduling,” says the insider. “They each felt that the other party wasn’t acknowledging sacrifices being made to spend time with the children.” The “productive” mediation session, notes the source, “seemed to really open the lines of communication.”…