According its user manual (http://www.usglobalsat.com/store/download/53/et312_ug.pdf
), the GPIO pin is meant for custom functions like an LED. If you are looking for a custom function like PPS, that would require custom firmware. A custom firmware will require an NRE charge, plus a minimum order of 1K pieces of ET-312.
Based on our engineers, revising the firmware for this specific request is doable. However, the output pulse of GPIO cannot be 1uS, and the output time delays about 400nS ~1uS.