Gisharel is a real person and was around in recent history. He is mentioned in WoT2 in the biography of Neria, an Inquisition agent from the Dalish (I presume you encounter in multiplayer). Apparently the more conservative, isolationist Dalish criticised Gisharel and had little respect for the Ralaferin clan because of him sharing Dalish lore and culture with human scholars. He was the Keeper of the Ralaferin clan immediately prior to the current Keeper.
Now my take on it is that he wanted to correct the view of the Chantry scholars that Shartan wasn't a real person. In another part of WoT2 the preface to the Canticle of Shartan says that people think it could have been an honorific given to any rebel leader among the elves and that there were several slave rebellions at the time of Andraste. So the idea is that if he was just symbolic or a conglomeration of more than one leader, then Andraste didn't owe him such a personal debt. However, the canticle of Shartan makes it clear that she did. It was his timely intervention, getting his archers to use fire arrows against an ice wall, that allowed troops to reach Andraste when she had been cut off. Now Sister Petrine, a somewhat controversial Chantry scholar for going against the party line, is the one who particularly highlights the part that Shartan played in Andraste's war. I wonder if this is because she had spoken with Gisharel. It is Gisharel who says of the Chantry "They forgot that Andraste called Shartan "brother"."
An interesting bit to the preface to the Canticle of Shartan is that it states there was an old elvish folktale that spoke of "a rebellion against tyrants led by a trickster warrior" (my italics). It seems to me that in the years of slavery the story of Fen'Harel got split between the folktale, probably remembered fondly by his followers but with the name omitted because not everyone felt the same way, and the tale of his betrayal of the gods, likely remembered by their followers, in particular their priests. I'm pretty sure the folktale does relate to Fen'Harel and what is more it acknowledges both his role as a rebel but also a trickster. Hence the two ways of interpreting his name.
The other thing I found interesting actually in the Canticle of Shartan is that it refers to the People, so must have been originally recounted by elves and that when a group of rebel slaves were fleeing their masters, they stopped by a hill called "The Lonely One" by the Tevinter. There they started to regret their actions and wonder whether they would not do better throwing themselves on the mercy of their pursuers. At which point "Shartan heard them", almost as if he hadn't been part of their group but had been somewhere nearby. Then "Shartan stood on the hilltop" and rallied them against the idea. Almost like he had been on the hilltop all the time and that is how he came to hear them. He helps them overcome their pursuers by stealth (trickery) before going on behalf of the People to check out Andraste's camp so only he will be harmed if they prove to be hostile. The picture that accompanies the text that is meant to show the elves ambushing the legion (of Tevinter) is actually one of wolves falling on some sort of horned beast.
I think this is a pretty clear pointer (along with the stained glass window of Shartan looking like Solas' tarot) that Shartan was a real person but he was a manifestation of Fen'Harel, the trickster rebel.