Yashasvi Jaiswal spot in test cricket is reserved because of his two double centuries in two consecutive test matches. I am not sure about Shubman Gill, he has already played 23 test matches and he is not very consistent in batting. He has scored 91 runs in last test which means he has secured his spot for this series at least.
It was England second biggest defeat in terms of runs and heaviest in 90 years. England should have scored at least 250 runs in second innings to avoid such humiliation.
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.
One recent example is another stylish opener, Mayank Aggrawal, he had the same success as Jaiswal when he made his debut in Australia in 2018, he impressed everyone and in his first 13 innings (10 tests) he was averaging 65 with 2 daddy 200s + 1 ton and 3 50s but in the next 12 test match he hit a rough patch (still had another daddy 100 and 2 50s under his belt in the rough patch) and now he's nowhere near the test squad.
Gill was about to face the axe after 2nd test but he made and secured his position, his recent 91 runs inning gave him an extra 4-5 tests long rope and maybe add the next Australia tour as well.
The point is 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.