If the Dalish were honest, they woudn't resort to turning another race into a convenient scapegoat for their misery.
Their tradition is based on a lie about humans and they are to blame for the blood on their hands.
As for Gaspard, the reason he dislikes elves is the same every Orlesian aristocrat does: they let people die at the hands of darkspawn during the Second Blight after they promised they would help protect the innocent in return to the Empire granting them lands to live (the Dales).
It should also be noted the Dalish hunt down every human that accidentally bumps into their clan while wandering into Orlesian territory. If you have any doubts, just remember what your Dalish friend tells your PC when you meet the lost humans in Origins near the ruins: we can't let them live or they'll tell other humans where our clan is.
IMO, the elves aren't victims. They're predators, just like everyone else. If they've been subjugated into slavery, so have countless humans, dwarves, Qunari and other unfortunate innocents captured by the Tevinter Imperium. However, having such a past is no excuse to teach your children to be xenophobes and legitimize the MURDER of humans based on a twisted sense of historical justice. Elves don't have that right. Not when they were once enslaved by their own kin - the Evanuris - in an empire built on slavery, blood magic and supremacist ideals - Arlathan - and who ultimately brought upon their own destruction.
Hating humans is just a convenient excuse to justify not taking responsibility for their actions in the Second Blight and wanting to live according to non-human values. If they wish to build their own culture, no problem. But an invisible line has been crossed when they validate that sentiment by turning humans into monsters at the eyes of elven children and blaming them of crimes they DID NOT commit.
In short, they LIE to their children and teach them it's alright to hate and kill a race, so long as it's the "right" race. That's dishonest and unethical. It's plain racism and xenophobia. No amount of suffering can justify such an atrocious philosophy. And what's worse: generations of Dalish elves have grown up with a lie and believing their ancestors are proud of them for it.