Ages ago (read: "in the early 90s") I found rules for converting gamebooks from a certain series of CYOA-type books into adventures to play with a GM and one or more players, instead of their normal solo use. These rules either came with or were published alongside a specific series, but I can't remember which one or any more about how the rules were presented. It may have been just a few pages in the back of one of the gamebooks, but it might have been its own small book.
What I do remember for sure is that it was one of the gamebook series that already incorporated chance into the fights, first by depending on stats that you rolled up at the beginning of your playthrough, and second by using die rolls (definitely d6s) during a fight page. The RPG conversion rules were definitely printed in hardcopy – this was long before the Internet was a common thing.
The RPG itself was fairly simple, little more than a task-resolution system and a how-to for using the gamebooks as GM'd adventures. Mechanically it wasn't too different than the d6-based systems built into these books for solo play.
What gamebook series was this RPG part of, and how was it published? Did it have a name, or was it just a "here are some rules" in the back of a gamebook? A good answer would give me enough information to track down a copy of the rules.
Best Answer
You might be referring to the Fighting Fantasy series of game books, starting with the Warlock of Firetop Mountain, which were later coalesced into a roleplaying system, also called Fighting Fantasy.
Googling for it now, I can see that the Fighting Fantasy Introductory Roleplaying Game was released in 1984, while an expanded Advanced Fighting Fantasy series was released between 1989-1994
Could this be the system you're looking for?