Based on Chapter-a-Day Volume 3 by Norman Berry.
Sheep gate: Lost sheep found and brought in.
Fish gate: Saved sheep become fishers of men.
Old gate: The truth of God never goes out of date.
Dung gate: Paul said “I count all things but loss...and do count them but dung that I may win Christ.”
See more complete thoughts here:
http://bibletruthpublishers.com/la138674