A gas mask that doesn't cover the entire face leaves the wearer vulnerable to the toxin entering via the eyes (at the very least blinding the wearer, as CS tear gas does). Most modern chemical warfare agents (that is, toxins developed after World War I) can affect someone even if the contact is just through the skin.
I'd like to see a full-body (MOPP4) protective gear model (no exposed arms, hands, neck, or head). Such a set of armor would have penalties to mobility and aim, but it would provide complete protection versus gas attacks.