Greetings,
When you open Hammer and you use your entity tool to place the entity "NPC_CITIZEN", you will notice that at the bottom of the properties panel (you can open this by double-clicking the NPC after it is placed on the map) you can change the model to the Gas Mask citizen.
After this you will want to go into the Flags tab in the properties panel and uncheck the box that says "Random head", because it will disable the gasmask citizen model.
If you need more help reply to my post and I will get pictures.