This is not a simple situation. It requires some thinking.
In general, permanent residency works exactly the same way, whether somebody has a 10 year GC or 2 year GC with pending I-751.
A GC holder cannot spend more than 6 months outside of the US on any given trip. Once outside for over 6 months, GC holder is breaking continuous residence. Once outside of the US for over a year, residency can be deemed abandoned, especially if you sell everything in the US, break the lease etc.
A conditional resident can file for reentry permit too. But the resident must return to the US for biometrics appointment, interview, etc. Otherwise I-751 will get denied.
If you know for sure you'll be leaving the US for several years, you can always withdraw I-751, file I-407 and leave the US. In the future, can apply for IR-1 visa again.
Travelling back to the US may be a challenge before getting immigrant visa again.