This is basically just plain gpg+email done via a browser extension. So no protection of metadata (headers), no indexing of body (by upstream provider).
So if you have (potentially sensitive) information in the subject, you can search on that, and on to/from/cc etc -- but not on email body. Only way to achieve that securely, would be to have an encrypted index (that might be stored in an IMAP folder, encrypted) and decrypt and search it locally. At that point you are doing something rather different from what gmail is (currently) doing, however (I believe mailpile does something along those lines).
Google is not a privacy company. How can they show you context relevant ads if they can't read your email? So this will probably never happen within Google for political reasons.
But even for technical reasons, it's probably a ways off. Doing encrypted search is a whole other problem on top of encryption. You can't just apply standard search out of the box. Given that Google is just now unveiling an encryption solution, I would not expect GMail (or any google service) to release services built on top of them for a while, even if they wanted to (which they don't).
Unless you are doing hashing and comparing a specific value exactly (e.g. how you would securely store a password hash in a DB) there is no general purpose text indexing package that I know of.
Will GMail be able to find those encrypted e-mails when I search for them?