She would never try to fight if the Holy Grail was to his back. The two possible objects of her desire, both before her – how could she resist?
This was the determination he had reached before Saber ever made her way to the grand music hall. All of it holds true, and he can see the raw thought playing out in her eyes. She has no choice, but he delights in seeing her decide.
Then, her golden sword shines brighter than his armor, brighter than the golden pools that bare too many Noble Phantasms to count.
"N—No!"
This is beyond the realm of anything she would do. He knows her – his bride to be. She will not destroy the Holy Grail in order to have some chance to resist his decision.
She cries out, as much lion as woman. She is angry and sounds as if she is in pain. The sound sears into his mind, and he feels cornered as if she is a lion. For a moment, all he can do is stare.
She seems to be resisting something, but he does not see how she can dispel her own attack without letting the deadly, beautiful light go free. Her small frame trembles as she cries out again, this time in words. Words not for him. He frowns, hardly able to hear them except for this.
He still has more weapons than she can hope to deflect. He can release them all, a catastrophe beyond recovery for her. He can kill her.
He does not want to do this.
Then, he understands. The third party who had entered the room, unnoticed by him, is her Master. Her Master is the cause of this, using his Command Seals to exact his will over her. Gilgamesh grits his teeth, ready to call down judgment upon this man for attempting to ruin his wedding.
Out of the corner of his eye, he sees her. The golden pools of light respond to his will, ready to turn upon the man, the mongrel. The man lifts his hand, another Command Seal carved their – red, glaring, and treacherous. Gilgamesh understands what he means to do and he has a single choice.
He can rain his treasures upon this mongrel and destroy him, trusting their aim to hold true through the perfect beam of light that he has seen issue from Saber's glorious sword before. Or, he can set aside his weapons and do something not considered before. Dozens of the little gates close, drawing the treasures they display safely back inside, protected from the destruction the mongrel above calls down.
Saber's blade swings down, and Gilgamesh has no choice but to dodge it. Only, he has not surrendered. The only counterattack deemed worthy of her at this point flies outward with a precise, familiar movement of his wrist as he throws himself aside. The weight of it pulls true in his hand – this weapon, more valuable than most of his weapons – this precious tool, bearing the name of his one and only friend, Enkidu.