In GTA V, there are over 60 hobbies and pastimes available around San Andreas. Some of these include Taxi Missions, Races, Parachuting, Triathlons, Sports, and more.
For this achievement, you only have to worry about obtaining ONE Gold Medal in the following activities:
- Flight School
- Off-Road Race
- Sea Race
- Street Race
- Shooting Range
For the Triathlons, Sea Races and Off-Road Races, you only need to get 1st place to obtain a Gold Medal. For the Flight School and Shooting Range, you only have to worry about getting a Gold Medal in one of the many challenges available. This achievement can be done with any character, so don't worry about keeping track of who does what.