Normally you would use a "soft" usart to generate the slower serial data in software.
So you would use the hardware usart for the USB, and make a soft usart for the GPS.
That's going to be very difficult with GPS data, Soft_Uart functions have no flow control or buffer so there's no way to know when new data has arrived and if you don't read the byte before the next one arrives you will lose it. As GPS data is a constant stream it will be very easy to run into problems.
In the Soft_Uart example, Rx is on pin 7 and Tx is on pin 6, it's in the help files.
Forgive me for asking but on the 4550 the USB lines are on RC4 & 5 and the Uart is on RC6 & 7. Why the conflict?