External GPS units will work with RWY, but the app does not have internal support for external GPS units.
If you plan on using an external GPS, you will need an app to enable RWY to interface with the external GPS input.
See Bluetooth GPS on the play store (https://play.google.com/store/apps/details?id=com.meowsbox.btgps&hl=en_AU&gl=US), or your GPS provider's software.