Gardner-based Heywood Healthcare has voluntarily filed for Chapter 11 bankruptcy protection, the organization announced Monday. Heywood Healthcare operates Athol Hospital, Heywood Hospital, Heywood Medical Group, Heywood Rehabilitation Center, Murdock School-based Health Center, The Quabbin Retreat, and Winchendon Health Center. “Core hospital services will continue to operate as usual,” Rozanna Penney, co-CEO of Heywood Healthcare, said in a statement Monday.