To unlock the Grenade Guru trophy in Anthem, you will need to complete three of the Grenade Launcher challenges which can be found by opening the Menu > Challenges > Weapons > Grenade Launcher
You will find four challenges available -- one for each of the game's three Grenade Launchers, and a final one for completing all three of the Grenade Launcher challenges.
The three challenges you will need to complete are 50 kills with the following three weapons:
Aftershock
Bombardier
Lurker