This is not at all what @Lil bear, and @OldUser stated. You can travel outside the US with a GC, and as long as the trip is not longer than 6 months, the question of continuous residence does not come up unless there are frequent short trips that add up to more than about 548 days outside the US in a 3 year period or 914 days in a 5 year period.
To the OP, you can certainly submit the N400 when that window opens as even though your I751 is pending as you are still an LPR. My wife did exactly that, and she had her I751 and N400 approved at the same time, and she actually never even received a 10yr GC as she completed naturalization and instead got her US passport.