You might be interested in an older post on another blog re: G1X (G series in general) about Manual mode and flashes;
"External Flash. I am extremely curious about external flash with the G1X. Flash is covered in Section 8; however, it does not satisfy my curiosity. One of my biggest complaints about the G series has been that its external flash capability appears to be intentionally crippled as compared to the Canon DSLR. Specifically, when a G series camera is placed in Manual exposure mode, the flash controls also become fully manual. Said a different way, there is no ETTL automatic flash control when a G series camera is in Manual exposure. Although I had high hopes that external flash on the G1X would be more like Canon DSLR, it more and more appears to be standard G series logic and controls for flash – in spite of the hopes raised by the wording on the external flash chart on page 204. I am afraid that the G1X does not meet my hopes for external flash."
This has been an issue I believe for the newer versions of the G series.