Green’s Dictionary of Slang

gummy! excl.¹

a mild oath.

1845		S. Judd Margaret (1851) I 189: ‘Gummy!’ retorted the woman. ‘He has been a talkin’ about me.’.
1855		T. Haliburton Nature and Human Nature I 218: Oh, gummy, what a horn!
1897		Kipling ‘Slaves of the Lamp — Part I’ in Complete Stalky & Co. (1987) 63: Gummy! What a sentence!
1904		G. Stratton-Porter Freckles 57: I’ll be having a book about all the birds. [...] Yes, by gummy! [DA].
1924		Kipling ‘The United Idolaters’ in Complete Stalky & Co. (1987) 148: ‘Gummy!’ said Beetle.