A crowd of more than 100 — including a strong JP contingent — gathered Monday to learn more about a proposed gas pipeline that would run beside an active quarry in West Roxbury. JP Forum and Theodore Parker Church hosted a "teach-in" about the project, called the West Roxbury Lateral Pipeline. Despite the name, some residents of JP and Roslindale are also wary of the proposal. "This is the biggest issue in southwest Boston," City Councilor Matt O'Malley told the crowd. The project has clearance from the Federal Energy Regulatory Commission but Boston-area politicians, prodded by grassroots efforts, are starting to turn against it.