There are a number of reasons why hazardous gas scenarios are among the most psychologically difficult of all hazmat situations (other variables as equal as we can get them). And the psychological state of a responder is just as important as their equipment.
First, many- in fact most- gases are invisible. Without the use of proper gas detection (and that always has a few variables), a hazmat responder is walking in blind to these situations. And this was a factor when I dealt with emergency situations with flammable or toxic gas. Even oxidizers and inert gases- when I was young and naive I used to think of these as "safe" gases- can affect our psychological state. There's just something about facing off with an "invisible enemy."
A friend of mine who served in Vietnam described how difficult it was to be plagued by guerilla attacks from Viet Cong who seemed to appear from nowhere, attack and then disappear again. This before he and many others came to grip with the tunnel systems in use. In fact, he told me that the psychological damage they infected was worse than the actual physical damage.
When gases leak, they disperses to cover a wide area. So, when I was in such a situation, in my mind I used to feel "surrounded" by invisible hazards. If it was a flammable gas, where would the spark come from that would cause my last day on earth? If it was a heavy oxidizing gas, as it dispersed would it come in contact with grease or a fuel source?
So, the first two challenges a responder has to deal with in regard to gas hazards are that most times the gas is invisible, and that it disperses quickly so that the responder will literally and mentally be "surrounded" quickly.