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