Here is the full list of all 50 MXGP 2019 - there are The Official Motocross Videogame achievements.
Beat another player's Ghost in Time Attack mode
Complete a Grand Prix in a Custom Track created by another player
Replace all Stock Components in the Season preset of your Sponsor bike
Win a Grand Prix at Valkenswaard in any race mode
Complete a race using the Helmet Camera in any game mode
Win a Grand Prix at Matterley Basin in any race mode
Save a Bike Setup for the first time
Complete all The Final Showdown challenges within the Playground
Complete all Speed Trial challenges within the Playground
Complete all Dexterity Trial challenges within the Playground
Complete all Gathering Trial challenges within the Playground
Win a Grand Prix at Teutschenthal in any race mode
Win a Grand Prix at Neuquen in any race mode
Win the MX2 Championship in Season mode
Win a Championship in Championship mode
Win a Grand Prix at Shanghai in any race mode
Win a Grand Prix at Afyonkarahisar in any race mode
Win a Grand Prix at Uddevalla in any race mode
Win a Grand Prix at Imola in any race mode
Win a Grand Prix at Lommel in any race mode
Win a Grand Prix at Loket in any race mode
Sign your first contract with an Official Team in Season mode
Win a Grand Prix at Semarang in any race mode
Win a Grand Prix at Orlyonok in any race mode
Start the replay at the end of a race in any game mode
Win a Grand Prix at Saint Jean d'Angely in any race mode
Win a Grand Prix at Agueda in any race mode
Win a Grand Prix at Mantova in any race mode
Win a Grand Prix at Pietramurata in any race mode
Win a race with the physics level set to advanced.
Get a Holeshot in any game mode
Sign your first contract with a Sponsor in Season mode
Validate 5 Custom Tracks
Compete on a Waypoint Path created by another player
Validate 5 Waypoint Paths
Reach Prestige Level 100
Win a Grand Prix at Kegums in any race mode
Complete a Championship in Season without changing Official Team
Sign a contract with the Monster Energy Kawasaki Racing Team MXGP team in Season mode
Sign a contract with the Red Bull KTM Factory Racing MXGP team in Season mode
Sign a contract with the Team HRC MXGP team in Season mode
Sign a contract with the Rockstar Energy Husqvarna Factory Racing MXGP team in Season mode
Sign a contract with the Monster Energy Yamaha Factory MXGP MXGP team in Season mode
Race a total of 408 km (253,4 miles)
Win the MXGP Championship in Season mode
Reach first place in all sessions in a race set at Full Weekend
Race 25 Km in free lap in the Playground
Finish first in a race under Rain in any race mode
Use the Rewind for the first time in any game mode
Take the podium 50 times in Grand Prix or Season mode