The most common and most likely answer to this crossword clue is the 8 letter word CHLORINE
C
H
L
O
R
I
N
E
Buddy explains!

The straight part
We know that "gas" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gas = chlorine.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CHLORINE mean?
CHLORINE noun- A toxic, green, gaseous chemical element (symbol Cl) with an atomic number of 17.
- A single atom of this element.