In modern English, the plural of gas is usually gases, and gasses is the simple-present verb. For instance, we might say that the more Bill gasses up his car, the more greenhouse gases his car emits. This isn’t a rule, though, and the plural gasses survives, appearing a small percentage of the time.
The verb is so rarely useful (and its senses outside the phrasal verb gas up are no fun) that the rare instances of gasses we find on the web are mostly variants of gases. There is one of these for approximately every ten gases.
Many scientists believe that carbon dioxide is one of the gases that contributes to the “greenhouse effect.” [Washington Post]
Rotten-egg smell is a symptom of both septic gas, which leaks in through the plumbing, and toxic drywall, which off-gasses hydrogen sulphide. [National Post]
The lights occur when ionised particles from the sun hit the Earth’s atmosphere and react with gases, in this case oxygen, which gives the red and green colours. [Irish Times]
He regularly gasses up a private jet on Cavs off days during the season and flies to New York for events. [ESPN]