The short answer: both gases and gasses are acceptable plural forms of gas. However, gases is much more commonly used and is often considered the standard form.
The explanation: in many cases in English spelling, final consonants like the s in gas are often doubled when an ending such as -es, -ed, or -ing is added to the end of the word. For example, the word nag becomes nagged in the past tense. This spelling—with a double consonant—makes it clear the word should be pronounced with a short vowel sound. Otherwise, it may seem as though the word should be pronounced with a long vowel sound as it is in words with a silent -e that follows a consonant (like cave, kite, and rope).
For example, doubling the r in bar when spelling barred (short vowel sound) avoids it being confused with the word bared (long vowel sound). Since gase is not a common word that may confuse things, we usually just pluralize gas as gases. The same applies to buses.
👉 Quick tip
A long vowel sounds like the name of a letter. (Cave has a long a sound.) A short vowel sounds like the “sound” the letter makes. (The a in apple is short.)
However, when gas is used as a verb and its tense is changed, the s is usually doubled: gasses/gassed/gassing.
The (very abbreviated) history: many of the spelling “rules” in English have long historical precedent. The practice of doubling consonants goes back to the earliest days of English. It was often done to distinguish vowel length (that is, to make it more obvious whether a vowel should be pronounced short or long) in words with very similar forms. That said, plurals like buses and gases are exceptions.
