Complex sentence: Chiefs fans were deeply divided over whether Gannon or Grbac should lead the team, and Gannon backers were none too happy when he was let go and signed with the Raiders in 1999.
Split into simpler sentences: Chiefs fans were deeply divided over whether Gannon or Grbac should lead the team. Eventually Grbac was preferred and was chosen to remain the Chiefs starting quarterback, Gannon was let go and signed with the Raiders in 1999.