Half life 2 episode 2.
This guide is intended for people who are carrying the Gnome.
Just before the race starts the game should autosave, Forget about the Gnome for now, drive as fast as you can the isn't to difficult. if you lose the race load the autosave and try again, When the achievement has unlocked load the autosave, This time Drive slowly and carefully taking care not to lose your Gnome.
I Hope this helpful