I just went through this with my GPS 175, GRT ARINC and Sport EX. This is what GRT sent me to get it working correctly.
GPS175 Settings:
ARINC RX Port 1:
Format: EFIS/ADC
Speed: Low
ARINC TX Port 1:
Format: GAMA Format 1
Speed: Low
RS232 Port 1: Aviation Output 1
The 175 does not have an RS-232 Fuel/Air Data (FADC) input like other Garmin IFR GPS units.
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 3 Rate: 9600
Serial Port 3 Input: GPS1 Aviation/MapCom
The Serial Port Output is not used for the 175 and can be used for unrelated functions at 9600 baud.
Using GPS1 for the GPS175 is recommended and sometimes required. Change any other GPS to GPS2.
If the ARINC module is connected to port 6, the settings would look like this:
ARINC Module Connected: Yes, Port 6
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 GPS175 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 GPS175 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.
Hopefully this helps.