Question is in the title.
Pretty self-explanatory: Urban Dictionary: [baby gravy]
I don’t know if this is a whoosh, but on the assumption it isn’t: it’s slang for semen.
Oh and my link is full of NSFW words, I really should have mentioned that sooner.
Not to be confused with diaper gravy.
the drippings are awfully fat to make good gravy.
That’s sick. Its not gravy made from babies, its gravy made from turkey drippings you put on babies before you eat them.
How is baby gravvy formed? How trukey get drippd?
Biscuits and baby gravy…YUM!
And chest potatoes.
Hey, if it was good enough for Jonathan Swift, it’s good enough for you and me.
As already said, it is a slang term for semen that men ejaculate when they reach orgasm. Also known as sperm, or seed.
Just adding this in case the OP is not a native English speaker, which I think is very likely.
Baby gravy=semen, seminal fluid, sperm. If I had to guess why it is because some varieties of gravy are white and opaque.
I like “spunk”, personally.
Whatever turns you on…
we always thought so. 
(For god’s sake, Puh-leez! This is the Straight Dope.)
I hate spunk.
Lou would be proud of you, KN