No, it isn't. A collective noun is a word such as 'group', 'team' or 'crew', which is a singular noun to refer to a set of individuals. Another example is 'flock', as in 'a flock of birds'. So we have, for example, 'a flock of birds', 'a group of friends', a gas of ..... No, 'gas' clearly isn't a collective noun! I wonder if you have misunderstood what 'collective' means. Are you thinking of 'gas' as a general term that can refer to various different gases? Oxygen, hydrogen and so on? If you are, the term you are looking for is HYPERNYM (definition "a word with a broad meaning constituting a category into which words with more specific meanings fall; a superordinate")