It's gamable if there is a single algorithm mandated by an authority - a single moving target can be gamed. But when you (eventually) have thousands of overlapping moving targets (different algorithms different settings per user) it makes gaming it futile. So the "best" way to game the system is just to do what you normally do - interact. Over time, you'll grow an organic reputation across all those target.
The number is a proxy, just like colors on animals and plants are proxies for their threat level. Human trust always beats computer trust, but the proxy helps in digital spaces when you want to decide who you care to interact with, or even more important - let into your community.