Gate of Babylon is a noble phantasm and trump card itself. It can summon virtually any attack or protection if not multiple of them. I would expect Fragarach to be able to do something like dumping Gil's treasury over him.
...No. Fragarach
can only be used in response to an enemy's strongest attack, at which point it strikes first and negates it. GoB is nowhere near Gil's strongest attack.
Also, both Archer and Kuro have been shown to know a hell lot of diferent noble phantasms which he tecnically shouldn't have ever seen. So I believe knowing an item from legend is more than enough to trace it. Shiro did trace Saber's first sword (also known as "Chops enemies into pieces 10 times better than Excalibur") and that dimensional dagger from memories and stories alone.
How does that change anything? They don't have any memories of Gae Dearg.
First, whatever hapened to Caster's demise? There was that big battle in episode 14, then episode 15 they're just talking about the aftermath! What did I miss?
Eh? He was inside Cthulhu when Saber blew it up.
You should always read the fine print on contracts.
In a Gen Urobuchi work? YES.
So, does this means that Rin walks around with the dagger used to backstab his father?
Yes. And the reason Kotomine goes down so easily in the Fate route is because Shirou attacks him with the same dagger and he's too busy being startled by irony to fight back.
Also, that's around 3 wizards and 1 cleric killed with perfectly mundane means. Does no magic user in fateverse walks around with lasting magic protections? No desesperate spells with their last breaths? Is Rin some kind of supe genius for the stuff she could pull off when in trouble?
Rin is a genius, yep. She only fails because of things she could have never predicted.