Once you get your Robostix programed and using the i2c buss there is no need to have it hooked up through the hirose (unless you need to talk through the serial lines) so you can connect the GPSstix directly via serial(using the hirose header) and hook up the i2c bus through the GPSstix breakouts of the SDA and SCL (i2c clock and data lines).
I'm totaly guessing but I don't think you should need any pull up resistors as long as the robostix is adequatly powered. But since the i2c buss from the robostix is runing at 5v and I'm guessing the GPSstix is runing at
3.3, your are going to have to adress that issue somehow.
You are definatly going to need to power both the robostix and the GPSstix though.