Green’s Dictionary of Slang

1. (orig. US campus) kissing (esp. in public and thus seen as excessive); also as adj.

c.1873		Schele De Vere Americanisms MS Notes 420: Gum-sucking = kiss[in]g in Ky. and Tenn; coupled with neck-sawing [DA].
1877		(ref. to 1854) Bartlett Dict. Americanisms (4th edn) 271: Gum-Sucking, a disgusting word, applied to the tendency of lovers, young ones especially, to carry their innocent endearments to an excess that displeases a third party. A friend informs me that he first heard it at Princeton College, in 1854, and thinks it may be a Jersey word.
1885		Bulletin (Sydney) 25 Apr. 13/1: When Victorian females – except while teething – take you for a professor, it will be time to – stay [...] Those gum-sucking maidens are thoughtful.
1900–10		Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 79: Gumsucking slang kissing or courting.

2. (UK) oral sex.

1988

(ref. to 1930s) S. Humphries Secret World of Sex 40: John Binns remembers [...] There were lots of cruder words for things like oral sex, I remember we called that ‘having a plate’ or ‘gumsucking’.