Yes. You can connect your Garmin G3X to ForeFlight. You can expect to receive GPS data and transfer flight plans (up to 63 characters long) to and from specific connected avionics units, such as the GTN 750.
Some aircraft are set-up with two G3X Touch displays, where one is the Primary Flight Display (PFD), and the other is a Multi-Function Display (MFD). In these cases, in order to transfer flight plans, the PFD must be directly connected to the GNS/GTN navigator via an RS-232 cable.
If there are two G3X Touch displays installed in the aircraft, you should only connect your iPad to one of the G3X's via Bluetooth.