Possibly a change in diet. Just like humans if dogs eat certain things it will give them gas. If the dog has gas all of the time you might want to try a different food.
odor
The dogs odor was terrible.
Ammonia :)
A few of them. Chlorine is one.
Helium is an odorless gas.
sulfur?
Yes due to its odor
This gas is radon.
A gas leak.
NO2 is a brown gas with a strong, unpleassant odor while NO is a colorless gas with a light, pleasant odor. NO2 is also toxic.
Give the dogs a good bathing. If the odor literally smells like dirty sweat socks and the dogs are scratching, it would indicate that they have the mange and need treatment as soon as possible or it will continue to spread.
Carbon monoxide