Since it was announced in 1987, if they mentioned the pronunciation it was soft G. The inventor and CompuServe would tell you it was soft G. CompuServe’s applications would tell you if soft G in their docs.
It’s even in the documentation of PNG which came out 7 years later that says soft G is correct in GIF, and they wanted people to pronounce PNG as “ping”, not “pinj”. (Yes, really)
JPEG (lemmy.world)
https://www.smbc-comics.com/comics/167225794620221228after.png