Hi bluetrain,
GM needs its own mame.ini, not one taken from other builds, you create it by running: groovymame.exe -cc
Then, you can't simply ask it to switch to a certain resolution, unless the monitor preset you use in mame.ini is capable of supporting that resolution. Do you see the point? Usually, with LCD monitors, you use the "monitor lcd" preset, which assumes the current desktop resolution is obviously supported, and sticks with it.
So, for instance, if you set your desktop to 1024x768, and use the "monitor lcd" preset in mame.ini, it will start all games at 1024x768.