Gas masks and NVG head straps are no longer compatible. Before 1.10, NVGs and Gas masks could be worn at the same time, however, now it is not as simple. You can not regularly equip a gas mask if a head strap is already on, but you can still equip a head strap even if a gas mask is already on. If you log out with a gas mask and head strap on, upon logging in, the gas mask will be gone. I have lost several of my precious NBC respirators figuring this out :(. A tactical helmet with NVGs works fine with gas masks, just not the head strap. I have a character that had a head strap and gas mask on prior to 1.10, and that character is able to log in and out with out it being deleted, but no other character can.