The phrase "narrow gate" means a difficult or challenging path or opportunity to achieve something. It suggests that one must face obstacles or make sacrifices in order to reach a desired goal. Full definition
Wrong in law is one thing and will mean an appeal should generally be allowed; whereas wrong in the exercise of discretion is a much narrower gate for the appellant to pass through. (bloomsburylawonline.com)
But if you're not from Toronto, and haven't been subjected to being herded through narrow gates guarded by dozens of police officers over the past few days, this is what it looks like: (slaw.ca)
How narrow the gate and constricted the road that leads to life. (religion.blogs.cnn.com)