Forza 5 is free this weekend on the Xbox One if you have a Xbox Live Gold membership, the free weekend start at 12:01am on September 4 and finishes 11:59pm on September 7.
The Xbox website has all the details, including the fact that you get to keep any Gamerscore earned:
“The Forza Motorsport 5 weekend will allow you to check out the full Forza experience with special in-game challenges, play against others online and build your achievements and Gamerscore. Throughout the weekend, you will have the chance to compete in a special multiclass endurance in-game race event, Grand Tour of Prague. Complete this endurance event and drive at least 130 miles in the race to earn a new title and badge for your Forza Motorsport 5 profile.
After the weekend, you’ll have the option to purchase the game online from the Xbox Store or visit your favorite retailer and continue playing. You will also get to keep your Gamerscore and achievements earned while playing, so invite your Gold friends to play and see who has the best driving skill.”
It’s nice to give people a weekend of the game to see if they enjoy it, it’s a good idea and I hope they extend this to other titles as well. However, it may only work with long games like Forza 5 otherwise people might complete the campaign in the free time period.