Nehra decided to send out a player to the middle with a message for Mohit at a crucial stage in the final over.
M S Dhoni-led CSK joined Mumbai Indians as the team with most number of IPL trophies.
'The amount of love I have received all over, the easy thing would be to walk away from here, but the harder thing would be to work hard for nine months and try to play another IPL,' Dhoni said after the final.
Chasing a revised target of 171 in 15 overs, CSK were home and dry off the last ball with Ravindra Jadeja hitting a four.
If the reserve day is also affected by weather conditions or is washed out, Gujarat Titans will be declared champions since they topped the league stage.
A win on Sunday will help CSK draw level with Mumbai Indians for the most number of titles (five).
GT also have the personnel to become the first IPL franchise to win the league in their first two attempts.
Gujarat Titans, who won the IPL title in their debut season last year, will meet four-time champions Chennai Super Kings in Sunday's final.
Gill, who brought his third hundred of the season, smashed seven fours and 10 sixes in his blistering knock. Among his 10 sixes, one shot stood out.
This was MI's maiden win over LSG in four matches. MI will take on defending champions Gujarat Titans in the second Qualifier on Friday.