I think it's better that the story not be told. Not every gap needs to be filled. There's always room for mystery, especially for characters who are so unique.
And I always suspected that Picard
and Guinan had another meeting besides the one depicted in "Time's Arrow"(a.k.a. "Data's Head"). That episode certainly didn't develop their special friendship referred to on the series. It had to be another occasion or occasions that formed their bond.