Google's end-to-end encryption plugin makes use of elliptic curve encryption, which is not supported but from GnuPG version 2.1 (and this is still a development version).
But you can go the other way round, and locally create a "normal" RSA key which you can then import to Google's plugin.
Anyway: as the key stays in the plugin, you're at least not giving the private key to Google, but they still can use it for arbitrary operations without you knowing it. What runs inside a web browser is under control of the web site owner, not yours. Better use a mail client (eg. Thunderbird) with a GnuPG addon (eg. Enigmail) and configure it to use your Gmail account – more comfort, more security, better privacy.
Just for the wording: the OpenPGP key version is in both cases v4, but elliptic curves are a rather new encryption algorithm not supported yet by stable GnuPG versions.