How can I directly connect a gps module to an xbee?

What voltage level does the GPS run at?

You can directly connect your GPS module to xbee as long as the voltage levels of GPS module and zigbee matches (i.e 3.3V) and if the xbee is configured to connect with the network. Considering the xbee is in transparent mode, whatever data is received on xbee UART(i.e your GPS module) will be transmitted to destination node.