Green’s Dictionary of Slang

gunga n.1

also gonga, gunger
[? gong n.1 ; but note Rudyard Kipling poem ‘Gunga Din’ (1892): ‘though I’ve belted you and flayed you...’, i.e. on the backside]

1. (Aus./N.Z.) the anus.

[US]Trimble 5000 Adult Sex Words and Phrases.
[US]B. Rodgers Queens’ Vernacular 19: the rectal opening, anus [...] gonga (obs, cf Sp ganga = a kind of prairie hen with the incubating rump of a brooding hen).
[NZ]McGill Dict. of Kiwi Sl. 53/2: gunga [...] anus, in phrs. ‘up your gunga’ or ‘stick it up your gunga’.
[NZ]McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

2. (US campus) an unattractive woman.

[US]Baker et al. CUSS.

3. (Aus./N.Z.) the vagina.

[NZ]McGill Dict. of Kiwi Sl. 53/2: gunga vagina.
[NZ]McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

In phrases

in one’s gunga (adv.)

(Aus.) directly, openly.

[Aus]D. Niland Shiralee 44: Then he told ’em in their gunga.

In exclamations