How To Install/Root Vegan-Tab GingerEdition on Viewsonic gTablet
Slight disclaimer: I don't own a gTab, so I'm not particularly familiar with it. That being said, rooting devices and installing custom ROMs is at least somewhat similar across devices.

I would start by taking a look at this thread over on XDA. It has links to a bunch of other threads with instructions on how to get a custom recovery (ClockworkMod) on your device, how to flash a ROM, and a bunch of other useful things. In particular, there is a pretty good guide for installing ClockworkMod v08, which would essentially get you to a point where you could follow the Vegan-Tab installation instructions.

Be advised that doing this will wipe all of the data on your device, so make a copy of anything you don't want to lose.