There is no such thing as reserved or permanent in the Indian team and it is also true for Jaiswal or other young batters, considering many are waiting in the wings.
Gill was about to face the axe after 2nd test but he made 100 and saved his position, his recent 91 runs inning also gave him an extra 4-5 tests long rope and maybe add the next Australia tour as well.
The point is, the Indian system is tough because of competition and when you get early success then you have to perform consistently for the next 10-15 tests as well otherwise someone else will replace you.
You are right there is nothing like a permanent position in the Indian team until a player plays like Rohit or Kohli. They have a proven track record and the cricketing world acknowledges them. I still think Jaiswal will have more opportunities than Gill as he has now joined the League of Legends after scoring back-to-back 200 runs. This is how Rohit Sharma became a legend and so did Virat Kohli, if he gets another double century as two more tests are yet to be played then he will cement his position as long as no one is challenging him in test cricket as an opener. Gill has been struggling it is visible and he might not get a berth in the team when Kohli is back unless he keeps on getting those runs as he has done in the last two innings.