If your site isn't for the monetary gain of the users, and they're not paying customers, then treat the complaints as a complement of sorts: people really like your site and miss it when it's down. You've succeeded in making a community which people visit frequently and it's important to their lives. Do apologize, do post a detailed blog post or other notice explaining what the problem was, why it happened, and how you intend to prevent it in the future. But you're under no obligation to compensate the users in any way. If your system is gamified do not give them additional point. There's nothing they've done to earn this reward, and it doesn't fall in line with the meaning of the points.
If your site is a service which users pay for, or a support forum which users depend on in order to get assistance using your product, compensation might be in order. If they've paid for the service monthly, you should have an SLA and you should issue refunds according to that SLA. If this is a support forum, consider offering credits for paid technical support services, coupons, or other discounts.
If you're running some sort of service which the users can actively earn money from and the service goes down, then monetary compensation may very well be in order. (E.G. if the gamification aspect includes monetary rewards) Again, you should have a good SLA in place, and stand by it.
You didn't make it clear if this is planned downtime. If it is, make sure that's covered by your SLA, announce it ahead of time and there's no reason to be sorry. All sites need planned maintenance.