GNX375 pins and settings are from customer notes.

The data pins are on the 3751 and 3752 connectors.

RS-232 connection
GNX375 Pin 3752-6 (RS-232 OUT 2) -- any serial input on EFIS
Example 1: GNX375 Pin 3752-6 (RS-232 OUT 2) -- HX/HXr/EX/10.1 pin A22 (Serial 5 IN)
Example 2: GNX375 Pin 3752-6 (RS-232 OUT 2) -- SX pin 30 (RX1)
Example 3: GNX375 Pin 3752-6 (RS-232 OUT 2) -- GRT ARINC pin 8 (Serial Input pass-thru on male DB9 on box)
GNX375 Pin 3752-31 (RS-232 IN 3) -- any serial output on EFIS, usually the same port as the input above
Example 1: GNX375 Pin 3752-31 (RS-232 IN 3) -- HX/HXr/EX/10.1 pin A3 (Serial 5 OUT)
Example 2: GNX375 Pin 3752-31 (RS-232 IN 3) -- SX pin 34 (TX1)
Example 3: GNX375 Pin 3752-31 (RS-232 IN 3) -- GRT ARINC pin 9 (Serial Output pass-thru on male DB9 on box)

A direct connection to the display is preferred unless you're out of serial ports.

ARINC connections
The GRT ARINC-429 connector (female DB9 on external module box or back of HX/HXr) will be referred to as the C connector here.
GNX375 Pin 3752-1 (ARINC 429 OUT 1A) -- GRT ARINC pin C1 (ARINC 429 IN 1A)
GNX375 Pin 3752-2 (ARINC 429 OUT 1B) -- GRT ARINC pin C2 (ARINC 429 IN 1B)
GNX375 Pin 3751-27 (ARINC 429 IN 1A) -- GRT ARINC pin C5 (ARINC 429 OUT A)
GNX375 Pin 3751-28 (ARINC 429 IN 1B) -- GRT ARINC pin C9 (ARINC 429 OUT B)

(Any other RS-232 input/output and ARINC-429 input/output can be substituted.)

ADS-B traffic and weather data:

For HX/HXr displays, you can use the RS422 mode of port 8.
Connect GNX375 pin 3752-5 (RS422 A) to HX/HXr pin B3.
Connect GNX375 pin 3752-20 (RS422 B) to HX/HXr pin B2.

For displays without RS422 inputs, you can connect the RS422 B pin to an RS232 input.
Connect GNX375 pin 3752-20 (RS422 B) to high speed serial input on EFIS.
Configure EFIS serial input for 115200 baud and "ADS-B" input.

GNX375 Settings:

ARINC RX Port 1:
Format: EFIS/Air data
Speed: Low

ARINC TX Port 1:
Format: GAMA Format 1
Speed: Low

RS232 OUT: Aviation Output 1
RS232 IN: ADC FMT 1

RS422 OUT: OPT LGCY ADSB

EFIS Settings in SET MENU, General Setup:

For units with external ARINC module: ARINC Module Connected: Yes, Port (select port it's wired to)

ARINC Receive Rate: Low
ARINC Transmit Rate: Low

Serial Port Rate: 9600
Serial Port Input: GPS1 Aviation/MapCom
Serial Port Output: Fuel/Air Data (Z Format)
Using GPS1 for the GNX375 is recommended and sometimes required.  Change any other GPS to GPS2.

If the GPS RS-232 is connected to the pass-thru port (pins 8 and 9) on the external ARINC module male DB9,
configure the serial port that the ARINC module is connected to as 9600 GPS1 Aviation/MapCom.

For example, if the ARINC module is connected to port 4 and the GNX375 RS232 inputs and outputs connected to its pass-thru port, the settings would look like this:
ARINC Module Connected: Yes, Port 4
For software before July 2022:
Serial Port 4 Rate: 9600
Serial Port 4 Input: GPS1 Aviation/MapCom
Serial Port 4 Output: Fuel/Air Data (Z Format)
The July 2022 and newer software will rename the pass-thru port to AMPT Serial Port:
AMPT Serial Port Rate: 9600
AMPT Serial Port Input: GPS1 Aviation/MapCom
AMPT Serial Port Output: Fuel/Air Data (Z Format)

If nothing is connected to the pass-thru port then the serial port input/output would be Off.

ADS-B input on HX/HXr
Serial Port 8 Mode: RS422
Serial Port 8 Rate: 115200
Serial Port 8 Input: ADS-B
Serial Port 8 Output: Off (not used)

ADS-B input on displays without RS422
Serial Port Rate: 115200
Serial Port Input: ADS-B
Serial Port Output: Off (not used)

Other settings in General Setup:

ARINC GPS Input: GPS1
This tells the EFIS to use the GPS data from the ARINC-429 connection when GPS1 is selected.

NAV Mode Source: Internal
The GNX375 does not have a built-in VOR/ILS receiver, so all mode changes must be done in the EFIS.

GPS1 Flight Plan Source: External
This tells the EFIS to use GPS flight plan data from the GNX375 RS-232 connection.

GPS2 Flight Plan Source: update this to match your GPS2, if any.
GRT GPS units should be Internal.  GPS units that transfer flight plan data should be External.

Virtual GPS2: Normally Off unless you know you want this feature.
If you have two GPS inputs this must be Off.

Settings in SET MENU, Primary Flight Display:

To enable the approach modes and PFD indicators, you must configure the ILS display:

ILS Type: pick Needles or Scales as preferred
When some other features are turned on, only Scales mode is possible.

Show VOR CDI on Localizer: No, unless you have a VOR input.

Show GPS CDI on Loc/GS: Yes
This displays the GPS approach deviations on the ILS indicators.
This is required for the GPS approach display and capture modes in the EFIS.