Can you connect Garmin GTN 750 to the Garmin G3X? If so, how?

Yes, the GTN 750 can certainly be connected to a Garmin G3X. You would need a GAD 29 to fully give this interface. GAD 29 enables your G3X to incorporate such advanced features as GPS steering, WAAS LPV vertical approach guidance and more. The GPS navigation data is provided to the G3X system via one RS-232 input and one ARINC 429 input for each GTN/GNS unit. The G3X system sends flight data and selected course information back to the GTN 750 via an ARINC 429 output. The G3X system uses an additional ARINC 429 input to receive VOR,localizer, and glideslope deviation from the GTN. The G3X Touch system can also remotely display and control the VHF COM radio, with them being controlled over the GTN 750 or their own front panel panels over the same RS-232 connection as navigation.