This data will be available on platforms like Ace MF or Morningstar, for which you will have to take a subscription, a bit difficult to find online. Also, you can use this https://mf.algotest.in/
For a particular mutual fund NAV data, try the AMFI’s NAV History report (can be downloaded for 5 years at a time, was able to pull data from 2006 onwards for multiple old Gilt fund from it like ICICI one)
Try the AMC website for even older data, like for example, able to pull data starting from 2001 for Kotak’s Gilt fund (Likely reason why data is available from January 2013 onward is because direct plans were launched by SEBI post 1st January 2013 onwards)