There is a microscopic world you can't see but your gourami sees and interacts with. It is probably getting a little supplimental nutrition by eating this off the plant. It is quite normal with many species of fish.
My gouramis pull almost every leaf off of my hornwort. It has begun more amusing than anything.
You should see in one of my betta tanks - I have duckweed - and apparently one of the bettas doesn't like the roots. So he chews them all off! There are a bunch of roots scattered across the floor of the tank - and the duckweed that's left is just the leaves!