Gas is a slang word for for a good time. The phrase was popular in the sixties and seventies in England.
From the Oxford English Dictionary
A gas c. Fun; a joke. Anglo-Irish slang.
1914 Joyce Dubliners 24 He told me he had brought it
[sc. a catapult] to have some gas with the birds. Mahony
used slang freely. 1962 E. O’Brien Lonely Girl ii. 20 ‘Let’s
do it for gas,’ Baba said. 1965 M. Kenyon May you die in
Ireland xxiii. 197 Gas is Irish for joke-gag, fun, having a wonderful time.
d. (Esp. prec. by indef. article.) Something or someone
that is very pleasing, exciting, impressive, admirable, etc.
Also attrib. quasi-adj. Cf. gasser 2. slang (orig. U.S.).
1957 J. Baldwin in Partisan Rev. iii. 357 Brand-new pianos
certainly were a gas. 1962 Austral. Women’s Wkly. Suppl.
24 Oct. 3/2 Gas, anything which is very good. 1963 Guardian
5 July 11/3 You can listen to his old records now and they are still a gas. 1963 L. Deighton Horse under Water xxxii. 124 ‘Yeah,’ he said, ‘got me an E-type Jag: Cambridge blue-wire wheels-it’s a gas.’ 1964 ‘J. Welcome’ Hard to Handle vii.79 I’m bound to say I thought it rather gas at the time. 1967 Melody Maker 27 May 10/6 Altogether an indefinite sound except for a gas build-up in the middle. 1971 Frendz 21 May 16/1 The Stones…were a screaming, speeding, sexy gas.