Grenades is a Hatred Generating Demon Hunter skill unlocked at Level 15.
Gas Grenades is a rune effect of Grenades which is unlocked at level 60. It changes the damage type to poison and leaves a DoT cloud of poison gas behind with each explosion.
Prior to the skill system revision, this rune effect was created by the Crimson rune.
Skill Design
A video showing this effect can be seen below. These visuals were obtained with an emulator from the beta client, and may not effect the final game graphics.