If it's an implementation of OpenPGP, then the algorithms are very trustworthy and have been vetted repeteatedly over the long term. Since it's a Chrome extension, it will be written in Javascript, so the source should be available to verify. It will also be intercompatible with every other OpenPGP implementation, and if those are backdoored, we're all doomed anyway.
The only reasonable attack vector an entity like the NSA would have (assuming the extension audits clean) would be to force google to update it to a corrupted version, which they presumably could have the power to do en masse or for individual users. I doubt that would go unnoticed for long though. And if it leads to a dramatic uptick in the adoption of secure email, IMO it's worth the risk.
