Gardiner actively lost this team games defensively, especially early in the season, and contributed nothing of value on the offensive side of the puck, which was supposed to be his great asset. But he got burned for breakaways constantly when trying to make offensive plays. He played better at the end of the season, but "better" is a relative term, and he did it in extremely sheltered ice time. He did not play well. Marc Staal is slow and useless these days, but he doesn't try to do to make aggressive plays like Gardiner does, so his mistakes don't lead to big breakaways on a nightly basis.
Gardiner makes the team worse every time he steps on the ice. Marc Staal is a passable 6D.