It depends on the types of position source you have in the GPS, and if you have Exemption 12555.
If you are SA-Aware equipped (newer GPS systems) and have Exemption 12555 - you have until 2025 to become SBAS equipped. NO preflight actions are required unless ADS-B is inop, when in that case call the command center for clearance to operate, like in the case of a no-transponder (both are inop aircraft).
If you are SA-On equipped, or SA-Aware without 12555 you are required to perform a preflight validation of the GPS coverage, and in all the flight planning system cases, there is a capability (if your carrier contracted it) to have the preflight validation ran in the flight planning system behind the scenes. If ADS-B is inop, call the command center for clearance to operate, like in the case of a no-transponder (both are inop) aircraft.
If you are SBAS equipped you're at the end state and you're done.
I was on the Equip2020 Taskforce coordinating the aviation response to the mandate from 2015-Dec 2019, so unfortunately I know way too much about it. The ADS-B MMEL Policy letter is being amended to reflect the relief the regulation provides and is in internal FAA coordination before it goes up for public comment, I wrote that too.