Second best solution after Startmail is Mailvelope. Extensions and addons are available for Chrome and Firefox. Fast and simple use with almost any webmail service, the list of supported services but I've tried unlisted services and it works just fine. You control multiple private keys and passphrases, and small file encryption.
Straight no frills PGP email. Clean interface and easy to use. Encrypt, decrypt, sign. Key management, import and export are easy. Your webmail will be filled with encrypted PGP messages and only the data, to, from addresses are searchable. Text in the email body is obviously not searchable.
Depending on the webmail service file encryption may be supported in the Mailvelope editor. If it's not, there is an option to encrypt small files under 25mb.
If you must use Gmail or Mail.ru or any other webmail - use Mailvelope. Mailvelope is the best solution if you really love gmail and other webmail services. It's smooth and fast. Encrypt, decrypt and sign in Chrome and Firefox. Encrypted emails in gmail and others make it hard to search for something that's in the body of the encrypted email, but it's worth the security when needed. Any PGP solution is unable to encrypt the subject, date and addresses in the header.
A great solution for encrypting and signing messages in your favorite webmail until you move to a more secure service.