Can I create paths to two external GPS antennae but use only one
If I understand correctly you want to make one PCB to fit both scenarios.
I would do this with a 0 ohm resistor, connecting to either one or the other antenna. Maybe there is already a coupling capacitor in the trace to the antenne then you could use this.

To minimize the impact on the 50 ohm impedance of the transmission line maybe you could place not 2 but 3 solder pads for the 0 ohm resistor or the capacitor. Then depending on which 2 (of the 3 pads) the component is soldered, antenna 1 or antenna 2 is connected.