Is there ANY possible way that you can use two serial devices (one GPS module, and one GSM module) to work with a single serial port? I am trying to send a SMS to a GSM module, and receive an SMS with location coordinates, but I only have the Pro Trinket 5V with one serial port.
Look into Software Serial ports for one of the devices. Or both if you need to leave Serial free for PC debugging.
How fast are the data rates? 9600? Software Serial is good for that.
It's in the IDE, Sketch:Include Library:SoftwareSerial