Here is the full list of all 31 Gravel trophies - there are 14 bronze, 10 silver, 6 gold and 1 platinum.
Unlock all trophies
Unlock all of the liveries in the basic game
Maintain "High Speed" for 40 consecutive seconds
Sustain a drift for at least 7 seconds
Obtain a time in Time Attack mode
Obtain all the stars in the Off-Road Masters Career mode in the basic game
Unlock all the vehicles in the basic game
Become the new Off-Road Master
Reach Level 50
Unlock all the Tracks in the basic game
Complete 10 online races
Complete all the special episodes
Finish first in a Speed Cross
Complete a race in every different location of the basic game
Reach Level 25
Unlock 20 new vehicles from the basic game
Perform at least 3 perfect landings
Perform a jump which lasts at least 3 seconds
Complete an online race
Complete a Smash-Up race, hitting only green signs
Unlock 10 new vehicles from the basic game
Complete a special episode
Reach Level 10
Finish first in a Head to Head race
Finish first in an Elimination
Finish first in a Wild Rush
Reach 142 mph (230 km/h)
Complete any track with a total of 6 seconds flying
Finish first in a Cross Country
Finish first in a Stadium Circuit
Obtain 3 stars in a Career event