It all depends on from where you are booting Linux. If you are booting
Linux from an SD card, then the simplest approach, is to mount the SD card
under another Linux machine and change the file (with an SD Card reader or
even a smartphone). If your image is in Flash, then you'll probably want to
get an SD Card and download a stock Gumstix image for your platform (to put
on the SD Card). Once you get the image and can boot from your SD card, you
can mount Flash and change it from the booted SD Card environment.
Currently, to use an American idiom, you have a chicken and an egg problem.
Assuming your booting from Flash, you cannot change /etc/inittab without
logging into your Gumstix system (it must be powered up and the USB console
attached). However, because getty is misconfigured, you can't log into your
Gumstix platform to change /etc/inittab. Therefore, you need an alternate
way to boot your Gumstix.