First of all, have you tried to unlock the gate with your jailhouse keypass? (Even though the keypass doesn't work on this gate, you need to try it to trigger the next bit of conversation.)
Assuming you've done that, you need to make one of your characters strong enough to twist the bars. The Cheese Flower key that you got in the Pit of Amnesia will open a door somewhere else in the game. This will lead you to some special equipment which will allow one of your characters to twist the bars.
Hope that helps.
