Menace (This creature can't be blocked except by two or more creatures.)
When Gilt-Leaf Winnower enters the battlefield, you may destroy target non-Elf creature whose power and toughness aren't equal.
The power and toughness of the target non-Elf creature are checked again as Gilt-Leaf Winnower's triggered ability would resolve. If that creature's power and toughness are equal at that time, the ability doesn't resolve.
Once an attacking creature with menace is legally blocked by two or more creatures, removing one or more of those blockers from combat won't change or undo that block.