There will be a few minor differences depending on how you've got your GPS flight plan set up, and most of it comes down to personal preference.
For example
Destination airport code is YABC
The destination is served by an NDB with the code ABC
The reference point on the DGA chart for measuring distance is the NDB
Option 1
Have the GPS set up to take you to ABC, with ABC as the final waypoint in the GPS flight plan. If you did this then your GPS shouldn't enter terminal mode and will continue indicating your course cross track position on the enroute scale. Once you fly over the GPS reference point (in this case the NDB) the GPS has nothing to sequence to and should continue indicating a distance to that point.
Option 2
Have the GPS set up to take you to ABC, with YABC as the final waypoint in the GPS flight plan. Once within 30 nm the GPS should enter terminal mode and the GPS will indicate your course cross track position on the terminal scale. Once you fly over the GPS reference point (in this case the NDB) the GPS will sequence and now indicate a distance to the final waypoint which is the ARP for YABC.
Option 3
Have the GPS set up to take you to ABC, with YABC as the final waypoint in the GPS flight plan. Once within 30 nm the GPS should enter terminal mode and the GPS will indicate your course cross track position on the terminal scale. Select OBS or equivalent mode with the desired inbound course. Once you fly over the GPS reference point (in this case the NDB) the GPS will not sequence to the final waypoint until OBS or its equivalent has been disabled and continue to give a distance to the GPS reference point (in this case ABC). Once OBS has been disabled the GPS should sequence to the final waypoint (in this case YABC) and now indicate a distance to the final waypoint which is the ARP for YABC.
Why would you choose one over the other and why should you care? Simple. Know your GPS, what it is doing, what it will do, and find either the way you prefer, or the SOP that your company employs. Keep it the same, and understand the information it is giving you.
Personally, I use option 3 as I may not decide until relatively late in the piece that I want to do an RNAV over the DGA and having the destination (YABC) as the final point in my GPS flight plan makes the setting up of that approach quicker and easier in a single pilot environment. I also like having the GPS operating at a higher level of precision during the approach, and once OBS has been set, I set the desired inbound course on the CDI which facilitates checking for "significant disparities".
Final note, is that if your GPS is set up such that it will engage terminal mode, this should have happened before you reach the IAF of the DGA. If you are flying with precision in the first place it should not make a major difference to your day.