If you're in active duty the I-751 won't matter, since to qualify for citizenship based on active duty you don't need even need to be a permanent resident.
But if you're filing based on 1 year of military service without active duty, you need to have valid permanent resident status on the day of the interview. Which means the I-751 must have been already filed if the interview is after the 2-year GC anniversary (because if the I-751 remains unfiled after deadline, the permanent resident status would be invalidated). They will decide the I-751 along with the naturalization application if the I-751 is still pending at the time of the naturalization interview.