The gas that would have been used is chlorine. It was delivered by releasing a bunch of gas canisters, and using the wind to drift it onto enemy lines.

"At around 5:00 p.m. on 22 April, the German Army released 168 long tons (171 t) of chlorine gas over a 6.5 km (4.0 mi) front, on the part of the line held by French Territorial and colonial Moroccan and Algerian troops of the French 45th and 78th divisions." (Edmonds & Wynne 1927, pp. 176–178.)

"German troops carried 5,730 gas cylinders, weighing 90 pounds (41 kg) each, to the front by hand. The cylinders were opened by hand, relying on the prevailing winds to carry the gas towards enemy lines. Because of this method of dispersal, a large number of German soldiers were injured or killed in the process of carrying out the attack." (Croddy 2002, pp. 143–144)

Chlorine gas is some really nasty stuff.

"Chlorine gas forms hypochlorous acid when combined with water, destroying moist tissues such as lungs and eyes. The chlorine gas, being denser than air, quickly filled the trenches, forcing the troops to climb out into heavy enemy fire." (Edmonds & Wynne 1927, pp. 177–178)

Many soldiers fled from the gas. Here's a first hand account from Anthony R. Hossack, of the Queen Victoria's Rifles.

"Plainly something terrible was happening. What was it? Officers, and Staff officers too, stood gazing at the scene, awestruck and dumbfounded; for in the northerly breeze there came a pungent nauseating smell that tickled the throat and made our eyes smart. The horses and men were still pouring down the road. two or three men on a horse, I saw, while over the fields streamed mobs of infantry, the dusky warriors of French Africa; away went their rifles, equipment, even their tunics that they might run the faster. One man came stumbling through our lines. An officer of ours held him up with levelled revolver, "What's the matter, you bloody lot of cowards?" says he. The Zouave was frothing at the mouth, his eyes started from their sockets, and he fell writhing at the officer's feet." (Hossak)

Works cited:

Military Operations: France and Belgium, 1915 Volume I: Winter 1914–15: Battle of Neuve Chapelle: Battles of Ypres, accompanying map case, Brigadier-General Sir J. E. Edmonds and Captain G. C. Wynne, 1927

Croddy, Eric. Chemical and Biological Warfare: A Comprehensive Survey for the Concerned Citizen. New York: Copernicus, 2002. Print.

Hossack, Anthony R. "The First Gas Attack." Published online at First World War.com. August 22, 2009

If you've done research, then you've probably seen this "dual diary." If not, you should check it out. Your grandfather most likely had contact with both of these men, considering he was their superior officer. Someone might be able to help you if you knew exactly when he died, but I did some research of my own and couldn't find anything other than what you already know.

Gas was vicious stuff. Even some of the soldiers planting the (90 pound/41 kg) gas cylinders were killed, since they had to be carried and opened by hand. Chlorine gas was introduced successfully at the Second Battle of Ypres, and would claim the lives of thousands. Deadlier than chlorine however was phosgene, which was produced in bulk by Germany, France, and Britain and accounted for most of the deaths by chemical weapons. By July 1917, the Germans introduced mustard gas, which is widely known as the chemical weapon of WWI, although in comparison to the aforementioned gasses, was the least fatal.

With that being said, it possible your grandfather was killed by being highly exposed to mustard gas, as it became more popular by the end of the war. Hopefully someone can help you find more info on him, although I do suspect his death might have occurred within either the Battle of the Lys or the Fifth Battle of Ypres.

Perhaps the most visceral account of a World War I gas attack is that of British Lieutenant Wilfred Owen, who was killed in action during the very last week of the war.

Wilfred Owen, "Dulce et decorum est" (1918).

Bent double, like old beggars under sacks,
Knock-kneed, coughing like hags, we cursed through sludge,
Till on the haunting flares we turned our backs
And towards our distant rest began to trudge.
Men marched asleep. Many had lost their boots
But limped on, blood-shod. All went lame; all blind;
Drunk with fatigue; deaf even to the hoots
Of tired, outstripped Five-Nines that dropped behind.

Gas! Gas! Quick, boys! – An ecstasy of fumbling,
Fitting the clumsy helmets just in time;
But someone still was yelling out and stumbling,
And flound'ring like a man in fire or lime . . .
Dim, through the misty panes and thick green light,
As under a green sea, I saw him drowning.
In all my dreams, before my helpless sight,
He plunges at me, guttering, choking, drowning.

If in some smothering dreams you too could pace
Behind the wagon that we flung him in,
And watch the white eyes writhing in his face,
His hanging face, like a devil's sick of sin;
If you could hear, at every jolt, the blood
Come gargling from the froth-corrupted lungs,
Obscene as cancer, bitter as the cud
Of vile, incurable sores on innocent tongues
My friend, you would not tell with such high zest
To children ardent for some desperate glory,
The old Lie; Dulce et Decorum est
Pro patria mori.

[Dulce et decorum est / pro patria mori:
'Tis sweet and fitting / to die for one's country. – Horace]

The story of Fritz Haber is a really interesting one. He pioneered the creation of industrial fertilizer (and was given the Nobel Prize for doing so), but later went into weapons and was intrumental in the weaponisation of chlorine gas and his company later created Zyclon B. He committed suicide after WW2. Radiolab did a great thing on him.