What does the expression *to kiss the gunner's daughter* mean?

No reference provided but it seems to come from the old Royal Navy tradition of lashing someone to a cannon to administer punishment using a cat o' nine tails. They would be tied across the gun to expose their back so their face would be below the level of the barrel. Therefore they could kiss the gunners 'daughter', a reference to the cannon mount.

While adult sailors received their lashes on the back, they were administered to boys on the bare posterior, usually while "kissing the gunner's daughter" (bending over a gun barrel), just as boys' lighter "daily" chastisement was usually over their (often naked) rear-end (mainly with a cane - this could be applied to the hand, but captains generally refused such impractical disablement - or a rope's end).

https://en.wikipedia.org/wiki/Cat_o%27_nine_tails