SDCARD2 is in reference to the microSD card. If you have a microSD card, just install the two file on the card and then reboot holding Vol+ and Power button until you see a screen with the word android on it. If you don't have a microSD card, then do not change the command file and load the two files on the root(main) directory of /sdcard on the Gtab. Then follow the same reboot process I mentioned.
Not sure if the update.zip file is borked or what. Try the stock file from this link. I used it successfully to restore my Gtab. Just put the files on /SDCARD instead of the microSD card "SDCARD2", which his instructions say.
Hmm....wish I could help you further, i don't use Clockwork.
I would suggest you post something over at xda-developers.com, under the Viewsonic G Tablet thread. They have alot more traffic there and users that use the clockwork recovery mode.