Disclosed are various exemplary embodiments of a retainer system for a ground engaging tool. In one exemplary embodiment, the retainer system may include a lock having a C-shaped portion defining a lock slot between two circumferential ends for receiving a portion of a support member to be locked with the ground engaging tool. The retainer system may also include a retainer bushing having an inner surface configured to receive the lock, where the lock may be rotatable relative to the retainer bushing and about a retainer axis between an unlocked position and a locked position. The retainer system may further include a cover piece configured to be attached to the retainer bushing for covering the lock slot of the lock when the lock is in the locked position.