If confronted by gate (that is closed) insert your ticket.

If it is the start of your journey, take the ticket from other point where it comes back out and go through before gates close.

If it is the end of your journey, most gates will retain your ticket. I.e. if you get a gate upon arrival, then you insert it gates open to go though, you wont' get given the ticket back.

Which is a reason for people to request a recipt if needed for expenses, etc. You can't rely upon keeping the ticket itself.