Which espruino hardware to read rs232 NMEA data from external GPS (Novatel Flexpack6)

While it doesn't appear there is a RS-232 module built

https://www.espruino.com/modules/

and it would take some digging to see if the onboard hardware UART could be used without the RS-232 handshaking CTS/RST, (maybe with software timers/timeout) it certainly would be possible to write a simple software equivalent using any of the pins described. (note: you may [should] need a level shifter - don't remember RS-232 output voltages)

Certainly there must be a common ground, Tx==>Rx, Rx==>Tx and depending on software both RTS and CST to initiate the handshaking. So, three, four or five connections. Chose a pin for each and set the mode for each pin first.

https://www.espruino.com/Reference#l__global_pinMode

If not well versed in RS-232 comm, creating one would be fun and a tremendous learning experience and made much easier using the post #3 logic analyzer described above.