Locked? You mean like with a padlock you can shoot off? Or a button to unlock it? Or ...?

You have to make it nonsolid for bullets to go through. If you made it a func_door_rotating (or similar) check the Passable flag in its properties.

The problem is then you can WALK through it too.

That is what CLIP is for. Add a CLIP brush to block players. You can tie it into the rest of the gate as part of the same entity, so the CLIP will swing open when the rest of the gate does.

CLIP does not block bullets.

Build the padlock as a func_breakable. Have it target the name of your gate.

Set your gate to Trigger Only (or somethine like that) which makes it so it can only be activated by a trigger event, not by the players directly.