Can such a system be gamed? Certainly, as can -any- system given enough time and effort, but it i s still better than nothing at all.
To be more specific, it is better than the system we have, which is easier to game. With the current system, everyone must trust the well-established venues. With the web of trust, everyone chooses their own trusted pointes to seed their web. This is very difficult to game unless you know your target's seed points in advance.