What's it about ? I kinda know that it's a bonus but I haven't really paid much attention to it despite being a long term freebitco.in member.
How do you get 10 GT by wagering 5 mbtc ?
You have to receive that special email and complete the "challenge" before the time expires in order to receive the rewards. As far as I remember, prizes go beyong 10 GTs... I think there were some RPs and lottery tickets as well to be collected.
If you wager the required amount of satoshis you will have the rewards credited on your account, just like it happens when earning prizes from WoF and RPs plus lottery tickets from multiplier and sports betting wagers.