FGT is still in the wraps shrouded in mystery. Kubo still didn't answer some questions pertaining to it and left it in the dust presumably to resurface it for another time whether in an interview, or in the manga, or perhaps never. Isshin seemed to be the only one who had knowledge of its existence long before Ichigo was ever aware about it. Although, the manga had never confirmed whether he was actually successful in obtaining the technique or not, and why he would make an attempt to obtain it to begin with.
That being said, FGT is the only reason why I'm inclined to believe that GT is a passed down technique from the Shiba Clan only. Any member of the Shiba Clan, or at least has their blood running within them, may use it instinctively without even knowing its proper name. As OM Zangetsu had once said before, Ichigo never knew the name of the technique despite knowing how to use it and then further explained that knowing the name of it and calling it out makes the technique even more effective and stronger. Again, I'm basing this off out of explanations from the very early stages of the series. This is all pure theory. Kubo might have alternate reasons to it now. He constantly likes to create and make things up along the way. An example of this was when he had decided at what it seemed like the very last minute to me that Aizen would be the main antagonist of the series, or Aizen's bull of planning Ichigo's whole entire life by basically connecting anything that would fit accordingly to his purpose that seemed just as plausible looking back at it now. That was well done, I admit.
