Gangga is a small island off the very tip of Northern Sulawesi. Unlike the walls of Bunaken and the muck of Lembeh, Gangga and its neighboring islands in the Bangka archipelago feature gently sloping soft coral reefs leading up to white sand beaches. Live-aboards from Bunaken and Lembeh will visit the islands on their way back and forth. There are a few dive resorts on Bangka Island, and apparently only one on Gangga.
One alternative to staying on Gangga or Bangka Islands would be Pulisan Jungle Beach Resort on the Sulawesi mainland very close to the Tongkoko Nature Reserve. The resort is within easy reach of the north islands like Bangka as well as the Lembeh Strait. There are also some interesting dive sites along the north shore of Sulawesi itself, quite close to the resort.