What was the time gap between the previous Buddha and Gautama Buddha? We all know our last Lord Buddha Was Gautama Buddha. My question is, before him who was the Buddha? And what was the time gap between that previous Buddha and Gautama Buddha?
It is difficult to give the exact time gap between the previous Buddha and Gautama Buddha. Both Buddhas came to pass in this great aeon.  This is a Special Kalpa – a Maha-Bhadra-Kalpa - largest time unit in Buddhism - unlike the previous ones, where five Buddhas in all will come to pass. Buddha says it's not suitable to count the exact length of the maha-kalpa in number of years. In the Anamatagga-samyutta — The unimaginable beginnings of samsara. In Samyutta ii, Chapter XV, the Buddha used the parables of the hill and mustard-seed for comparison:

Suppose there was a solid mass, of rock or hill, one yojana (eight miles) wide, one yojana across and one yojana high and every hundred years, a man was to stroke it once with a piece of silk. That mass of rock would be worn away and ended sooner than would an aeon.
Suppose there was a city of iron walls, one yojana in length, one yojana (eight miles)  in width, one yojana high and filled with mustard-seeds to the brim. There-from a man was to take out every hundred years a mustard-seed. That great pile of mustard-seed would be emptied and ended sooner than would an aeon.

In the Digha Nikaya in Mahāpadāna sutta, the Discourse of the biography of the Buddha it is said that:

&quot;In the time of the Lord Buddha Vipassi the life-span was eighty-thousand years; in the time of the Lord Buddha Sikhi seventy thousand; in the time of the Lord Buddha Vessabhu sixty thousand; in the time of the Lord Buddha Kakusandha forty thousand; in the time of the Lord Buddha Konagamana thirty thousand; in the time of the Lord Buddha Kassapa it was twenty thousand. In my time the lifespan is short, limited, and quick to pass: it is seldom that anybody lives to be one hundred.”
At the time of the next Buddha, the average lifespan of a human is going to be eighty thousand years. (In the present day it is around 60 -80 years).