Perhaps it happened like this:
King Leo's knights were enforcing a royal tax, and came into Gilberts village when he was a teen. His parents had no money to pay the tax and when they were hauled off to the dungeons, Gilbert fought back. As a punishment for this rebellion against the King he was sentanced to death. However, because the King was feeling "lenient" he only lost his eye.
Gilbert wanderd the kingdom until he Joined up with the bulls, for only one thing...(you guessed it) REVENGE.
This would explain the sour look on his face.