It's something you can only do in scripting. The command is HideHuds(). To show them again use ShowHuds(). If you are unfamiliar with how to use scripting in Game Guru, go to the Scripts forum and poke around for help and examples.
System Specs: OS - Windows 7 Home Premium 64-bit SP1, CPU - AMD Phenom II X4 945, 3.0Ghz, RAM - 8Gb DDR3, GFX Card - 2048MB NVIDIA GeForce GT 640, FPSC-R Version - Beta 1.0085