Simply, no (IMO).

Gas masks or APRs (air purifying respirators), APRs are good to protect the user from a know toxin (or range of toxins). APRs are not viable protection when the toxin is unknown, or in an oxygen deficient atmosphere. Effective duration of protection provided by a cartridge(s) will vary depending on the concentration of the toxin(s) and respiratory rate of user.

Someone who lives in an environment where they know that they can be exposed to a specific hazard could find owning a gas mask helpful. For example, soldiers are issued gas mask that are designed to protect them from known chemical and biological weapons.

Now, one could argue that one of these military gas masks would be good to have. However, when would you don it? When would you take it off? Without detection equipment it how useful would it be?

If owning one makes you feel more secure, than go for it… but I bet you could find much better things to buy with the $300+...