Green’s Dictionary of Slang

gas adj.

[gas n.1 (3)]

1. of objects or people, enjoyable, exciting, funny.

[US]C. Connors Bowery Life [ebook] ‘Extract of what?’ asked de wise guy, showing his crockery wid a gas laugh.
[Ire]D. MacDonagh Happy as Larry Act I: She’s a gas looking widow all right.
[Ire]B. Behan After the Wake (1981) 54: The gassest little ex-Dublin Fusilier in the street.
[Ire]J.P. Donleavy Ginger Man (1958) 181: You’re a gas man.
[Ire]P. Boyle At Night All Cats Are Grey 124: It’ll be right gas. [Ibid.] 162: He’s a gas man, the bould Willie.
[Ire](con. 1920s) L. Redmond Emerald Square 95: You’re a gas man.
[Ire]R. Doyle The Van (1998) 404: He was some tulip, Bertie; he was fuckin’ gas.
[Ire]G. Coughlan Everyday Eng. and Sl. [Internet] Gas (a): funny.

2. impressive, extraordinary.

[US]Rigney & Smith Real Bohemia xiv: gas the best, the most.
[Ire]R. Doyle Snapper 139: Michah’s ma ate coal when she was havin’ him. [...] That’s gas.
[Ire]P. Howard Curious Incident of the Dog in the Nightdress 259: I had a chat with oul’ Fehily. A gas character altogether.

3. see gassy adj.1 (2)