Have a gravity drain, does not seem to be draining properly.. bottom of pit is20 3/4" ..the water then run 6 feet to city drain which is at 21.75".. is this enough of a slope

If I understand your question you're asking if a pipe located in a pit at 20 3/4" will drain to a city drain located at 21 3/4". Correct?
The answer is that you have back fall on the line so the pit will only drain down to 20 3/4" before it stops draing leaving a inch or so in the pit. You either raise the pit or lower the drain it connects to,
The correct fall for a 2" or 3" drain line 6 feet long is 1 1/2 inches total.
Are your figures correct?
Good luck, Tom

Good point Tom...

You have to tell us how high is the outlet from the pit, not bottom of the pit. Can you post an elevation drawing of your set up ? Back to you...