Creating graphical user interfaces (GUI) for stereoscopic 3D (S3D) games is a difficult choice between visual comfort and effect. We present a S3D Game GUI Design Space and a list of S3D-specific attributes that emphasizes integrating visually comfortable interfaces into the game world, story and S3D view. To showcase our approach, we created two GUI concepts and evaluated them with 32 users. Our results show quality improvements for a combination of bottom position and visual attachment for a menu. In a referencing interface, placing the reference near to the target depth significantly improved perceived quality, game integration, and increased presence. These results confirm the need to create S3D GUIs with perceptual constraints in mind, demonstrating the potential to extend the user experience. Additionally, our design space offers a formal and flexible way to create new effects in S3D GUIs.