A 66-run fourth wicket stand between skipper Eoin Morgan (47 not out off 40 balls) and Rahul Tripathi (41 off 32 balls) provided the base for the Kolkata franchise to reach the target with 3.2 overs to spare.
Glenn Maxwell and Sheldon Cottrell are likely to be released ahead of the 2021 edition, according to a team source.
Kings XI Punjab failed to seal a place in the play-offs after losing to Chennai Super Kings.
Opener Ruturaj Gaikwad scored his third successive fifty as CSK overhauled the target of 154 in 18.5 overs to dash Kings XI's hopes.
The IPL did not specify the exact incident for which he was penalised but it is understood the Jamaican was penalised for throwing his bat away in disgust after getting out for 99 against Rajasthan Royals.
The win took Royals to fifth spot with 12 points from 13 games, while KXIP remained in fourth place with 12 points from 13 matches.
Gayle, 41, is way ahead of his nearest competitor, fellow West Indian and Mumbai Indians player Kieron Pollard, who has hit 690 sixes in T20 format.
Casing 150 on Monday night, Gayle, who was overlooked and was also ill in the first phase of the IPL, showed why he is called the 'Universe Boss' during his innings of 51 off 29 balls against KKR.
Mandeep and Gayle were associated in a 100-run partnership for the second wicket that took them to the brink of the big win.
Needing 17 off the last two overs, SRH could manage only four runs for the loss of five wickets.