What is the different between 'jag hade givit dig tid' and 'jag hade gett dig tid' (both are accepted answers)?
I don't know but my guess is "givit" is more old fashioned and reltaes to the infinitive form "giva" and the present "giver"
That's correct, givit feels more old fashioned but there's no difference in meaning, it's just an older form of the same verb. (the infinitive and present sound even more old fashioned).
Is "Jag hade gett tid till dig" also correct?
No, that sounds like 'I had given time to you' as if you had wrapped it in a package and given it as a gift somehow.
Why not 'Jag hade gett ni tid'?
ni is the subject form but since you give something to someone, you need to use the object form er instead: Jag hade givit er tid.
Compare: "You gave I/me time" in English.
Why 'gång' is not allowed here ?
gång is time as in one time, two times, three times...