Green’s Dictionary of Slang

git-down n.

(US) the start, the beginning; the crux of a matter.

[US]S. Morgan Homeboy 85: When they finally got down the gitdown, she was afraid she’d have to use Rustoleum.
[US]‘Touré’ Portable Promised Land (ms.) 60: Ya got ta git-down wit the git-down.

In compounds

git-down time (n.)

(US) the start of a prostitute’s working ‘day’, when she gets down to business.

[US]Milner & Milner Black Players 30: After the git-down time, me and my partner goin’ to the jam house for a little blow.