You don't need a gas mask to make use of this British Army issue gas mask bag. It comes with two generous side pockets, an inner side pocket, molle straps, a shoulder/waist strap, and two elastic inner loops under the lid. It's a truly decked-out piece of gear with many possible uses.
The main compartment measures 4" x 12" x 9" and the side pouches each measure 4" x 9" x 2".
*The thigh strap in the pictures may or may not come with this bag,