Plain English Meaning
A stretch of a published route where the navigation signal drops out, so the pilot temporarily has no reliable course information from that station along that segment.
Full Definition
A segment along an instrument route or procedure where the signal from a ground-based navigation aid is not strong or reliable enough to provide continuous course guidance, leaving a portion of the route without usable navigation information from that aid.
Context Anchor
Seen in instrument procedure and route design discussions, especially where the FAA explains places where continuous navigation guidance may not be available.
Derivation
A 'gap' here means a missing piece — a stretch where the expected navigation signal isn't there. The phrase describes exactly that: a in the otherwise continuous course guidance the pilot is meant to receive along a route.