For those of us who are Tier 8 (which skips the Tier 6 gold medal)
Here are some I have found so far:
Add 100 cars to your garage
Complete 4 Career Volumes
Drive for 500 miles (This was my first gold)
Reach Driver Level 30
Complete 12 Career Car Divisions
Lead 250 Laps in a Race
Earn 100 Race Wins
Complete 50 Showcase Events
Use 800 Mods
Gain 250 Perfect Passes
Reach Tier 6 in Forza rewards (may be less?)
Will update when I find more, but here is a start.