My reading of the text sent to me by e-mail is that it refers to a small metal plate, set into the front edge of the player, against which a magnet, fixed to the flap, will close with a 'snap', thus holding the flap closed.

I imagine the expression 'gag bit' comes from the analogy with a metal 'keeper' across the poles of a traditional 'horse'-shoe magnet, but I've not found any references where 'gag' is used in this context. It might be worth consulting a kitchen furniture manufacturer to see what they call the equivalent plate on the doors of fitted cupboards ...

Or, perhaps, gag is a typo for 'gap' .... ?