I didn't know what you meant by G2 and G-CUT. CUT isn't on my FW-900 menu, unless it's named something else there, like Center and Size. GAIN neither... unless you're talking about Brightness and Contrast levels.CUT and GAIN, are adjustments you can do in the main menu, they have nothing to do with WinDAS. They're simply brightness and contrast for each color.
When you decide to go deeper and do the WinDAS adjustment, there is a guide by Spacediver here on Hardforum.