Gag clause would only be enforceable if it was agreed and specifically mentioned in the settlement agreement.
Usually the person receiving the settlement has the upper hand - as the settlement is effectively a concession that the claim is valid. If they dont want a gag clause, then they should ask that one not be inserted.
One unusual settlement I've come across is where a gag clause was inserted by one of the plaintiffs in a multiple plaintiff case where the particular plaintiff, for personal reasons (ex-wife) did not want the settlement disclosed and said he wouldnt allow the case to be settled unless all parties agreed to a gag.