I am just considering the fact after 10-15 yrs would it be possible to still read those files?
I use it (gnupg) for some of the data in my org files. It's just long term use, I'm concerned about.
If your concern is in the long term, wouldn't you want to put your trust in the most mature and long lasting project today (i.e. gpg)? Nobody can see the future, but given that gpg is open source and has been actively developed for over 20 years I feel comfortable trusting it to continue to exist.
That phenomenon has a name and a Wikipedia page (probably called "X's Law") but I forget what it's called. But the point is: the longer something has been around, the longer you can expect it to be around (given that you know nothing else about it). Cf. the age of a mountain vs an individual butterfly and your prediction of how long you expect it to be around for.