How do these cards play together?

Poorly.

Longer version: Gunner allows a primary attack. Outrider prohibits primary attacks. Can't beats Can, so the Gunner attack will not happen as long as your cannon is in place.

As a waste of 5 points.

Gunner only works if you have a primary weapon left, so you'll be guarding against the very minor possibility of drawing a Munitions Failure crit-card.

Outrider title makes a cannon which a secondary weapon your only weapon to use to fire outside your firing arc. Gunner is only used with primary weapons. You're just throwing away 5 points.

If you suffer Munitions Failure your Gunner joins the action.