Yes and no.

Guns must be in locked hard-side containers but those don't need to be distinctive--the rule doesn't say the suitcase must be a locked hard-side. You can lock the gun in a suitable case and then put it in an ordinary suitcase. If I were flying with a gun that's certainly how I would do it as the non-TSA locks would be a giveaway to any thief as to what the contents are.