Assuming "GB" isn't a typo...!
I'm afraid SQL Multi Script needs to load the entire script into memory before executing it. On a 32-bit machine, each process is limited to 2GB of memory space, even if the system has more physical memory installed. [Edit: I've just checked and SQL Multi Script is currently set to run as a 32-bit process, even on a 64-bit operating system, so upgrading to Vista x64 won't help here]
We tested up to about 500MB scripts, if I remember correctly, but that's a couple of orders of magnitude smaller than what you're attempting.
Out of interest, what are you trying to do that results in such a gigantic script?