The Grenade Launcher was first available in Tomb Raider 2. It was a very effective weapon, you could bring down most opponents with one shot. It was also effective against the Dragon or the Stone Warriors (in the PlayStation Version you could destroy them with it, before they came to life)
Alas in Tomb Raider 3 the weapon lost its precision and wasn't as useful any more.
In Tomb Raider 2 you only had standard grenades for the grenade launcher:
Since The Last Revelation you could choose between three types of Ammo.
Normal Ammo, Super Ammo and Flash Ammo.