Here is the full list of all 30 Gravel achievements worth 1,000 gamerscore. It takes around 15-20 hours to unlock all of the achievements on Xbox One.
Unlock all of the liveries in the basic game
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
Unlock all the Tracks in the basic game
Reach Level 50
Maintain "High Speed" for 40 consecutive seconds
Complete all the special episodes
Complete a race in every different location of the basic game
Perform at least 3 perfect landings
Complete 10 online races
Reach Level 25
Complete an online race
Sustain a drift for at least 7 seconds
Unlock 20 new vehicles from the basic game
Perform a jump which lasts at least 3 seconds
Complete a Smash-Up race, hitting only green signs
Complete a special episode
Finish first in a Head to Head race
Reach Level 10
Unlock 10 new vehicles from the basic game
Finish first in an Elimination
Reach 142 mph (230 km/h)
Complete any track with a total of 6 seconds flying
Finish first in a Speed Cross
Finish first in a Cross Country
Obtain 3 stars in a Career event
Finish first in a Stadium Circuit
Obtain a time in Time Attack mode
Finish first in a Wild Rush