It is THE narrow gate, as it is compared to the wide one, being two distinct paths to follow.
"Broad & spacious is the road leading off into destruction, but narrow & cramped is the road leading to life" (best I can remember!) Matthew 7:13
Meaning: The Christian lifestyle is not easy to follow and requires effort "narrow gate/road" - it is much easier to 'take the wide road/gate'.
- For Teachers