Here is the full list of all 30 Gravel achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Perform a jump which lasts at least 3 seconds
Complete a Smash-Up race, hitting only green signs
Complete a special episode
Unlock 10 new vehicles from the basic game
Finish first in a Head to Head race
Reach Level 10
Finish first in an Elimination
Finish first in a Wild Rush
Reach 142 mph (230 Km/h)
Finish first in a Cross Country
Complete any track with a total of 6 seconds flying
Finish first in a Stadium Circuit
Obtain 3 stars in a Career event
Finish first in a Speed Cross
Complete all the special episodes
Reach Level 25
Reach Level 50
Maintain "High Speed" for 40 consecutive seconds
Unlock 20 new vehicles from the basic game
Unlock all the vehicles in the basic game
Perform at least 3 perfect landings
Unlock all of the liveries in the basic game
Complete an online race
Complete 10 online races
Complete a race in every different location of the basic game
Sustain a drift for at least 7 seconds
Obtain all the stars in the Off-Road Masters Career mode in the basic game
Become the new Off-Road Master
Unlock all the Tracks in the basic game
Obtain a time in Time Attack mode