Mass-market head mounted displays (HMDs) are currently attracting a wide interest from consumers because they allow immersive virtual reality (VR) experiences at an affordable cost. Flying over a virtual environment is a common application of HMD. However, conventional keyboard-or mouse-based interfaces decrease the level of immersion. From this motivation, we design three types of immersive gesture interfaces (bird, superman, and hand) for the flyover navigation. A Kinect depth camera is used to recognize each gesture by extracting and analyzing user's body skeletons. We evaluate the usability of each interface through a user study. As a result, we analyze the advantages and disadvantages of each interface, and demonstrate that our gesture interfaces are preferable for obtaining a high level of immersion and fun in an HMD based VR environment.