First off, I would not use a Zigbee mesh based module for this. I would use either the 900 HP in Point to point mode or the XBee 802.15.4 modules.
Next, if the GPS receiver is a 5V TTL device, I would use a 3V to 5V level shifter such as one from Spark Fun to connect the UART of the GPS Rx line to the Tx line of the Xbee module.
Then by using Transparent mode on the two XBee modules, you should receive your data.