For this achievement you must earn a gold medal in 20 career races. This does not mean you have to come 1st in all of those races and it does not mean that they have to be 20 different races.
I would recommend going to the racing league, and from there going to the modern Indy series.
It is the second race in the series and may already say that you have gold in it if you have completed that series.
The race is 5 laps long and requires you to come in at least 3rd place.
Once you finish the race just hit restart on the post-race screen to avoid all of the loading screens.
Do this 20 times and the achievement will pop after you have finished it for the 20th time.
The reason for choosing this event is that this will count towards your miles needed for the Grand Prix Veteran achievement.
If you already have the achievement then head over to the Sport compact events and to the Production rally series where you will only have to complete 3 laps to finish