Better Gabriel Image
Gabriel is a Non-Physical Entity
Gabriel Miller showed absolutely no sign of evading.
Still smiling thinly, he simply opened his arms wide.
Eight colors of light pierced into his bluish darkness-wrapped
body.
Not missing the opportunity as his upper body trembled,
I swept
the sword in my right hand towards the man's torso and thrust
the sword in my left hand through his chest. Slimy darkness
splashed and left a chill where it brushed across my skin.
Right afterwards I flew to pull back the distance and whirled
around again.
What my eyes ended up capturing was—
Gabriel slowly
drawing the spilled, amorphous darkness back
towards him, and turning around as if nothing had happened. The
jacket covering his body was unscathed.
(Sword Art Online - Volume 18, 130)
Simple. Kirito can't hurt Gabriel because he's made out of a non-physical material. Notice the "darkness" it emphasizes, and look at the image I posted.
Kirito clashes with Gabriel
The greatest, most final enemy, Gabriel Miller, entered the attack
range of my sword.
With my arms charged full of power I had never felt before, I
unleashed my most practiced, most reliable dual wielding Sword
Skill.
Starburst Stream. A 16-hit combo attack.
"WHO...OAAAAAAAAA!!"
My sword, permeated with starlight, shot forward with a blinding
trail.
At the same time, Gabriel's six wings and one blade came
swooping down from all sides.
Each mighty clash between light and void created vast flashes of
light, causing the world to tremble.
Faster.
Gotta go faster.
"OHHHHHHHHHHH!!"
I roared as I assimilated mind and body, accelerating to the
extreme and swinging my swords.
"NULLLLLLLL!!"
Gabriel was howling too, counterattacking with all seven blades.
10 hits.
11 hits.
There was no more rage, no more hatred, no more bloodlust in my
chest. All that was left pervading my body was the colossal power
of the prayers, driving me.
12 hits.
13 hits.
—Take in...
14 hits.
—the light of...
15 hits.
—the spirits of everyone of this world!! Gabriel!!
The final 16th hit was a full-motion overhead slash from the left, a
beat slower than him.
Certain of his victory, Gabriel's inhuman eyes narrowed slightly.
Moving a hair faster than the slash I had put all of my might into, a
black wing protruding from the enemy's right shoulder sheared
off my entire left arm.
The light-filled limb burst into pieces and scattered, leaving only
the Blue Rose Sword floating in the air.
"LLLLLLLLLLLLLLLLLLL!!"
(Sword Art Online - Volume 18, 162 and 163)
Shortly after the attack, Gabriel survived, so Kirito switched with Eugeo, as shown in
this image.
Accompanied by high, loud laughter, the void blade in Gabriel's
right hand hurtled downwards, crackling with black lightning.
Psht.
With the echo of that dependable sound, two hands that were not
my own gripped onto the hilt of the Blue Rose Sword hovering in
midair.
The Blue Rose Sword blocked the void blade firm and true.
Clutching the sword, Eugeo looked to me, flaxen hair swaying.
"Come on — now's the time, Kirito!!"
"Thanks, Eugeo!!"
I shouted back in clear tones.
(Sword Art Online - Volume 18, 164)