Have you considered the Standard Horizon GPS Smart Antenna
For example, Defender lists it for USD 120: Standard Horizon GPS Smart Antenna
It comes with cable that has a connector for plugging into a plotter, but that could be cut off leaving a pigtail to wire directly into the GX2150.
Since that antenna
is an RS232
feed of NMEA
0183, you could splice the pigtail for a couple of different applications, such as to both the GX2150 AND a computer, though that would also need an RS232-to-USB adapter.
Mount it inside, outside, wherever it works for your boat