That didn't take long, did it? After the first Nexus devices shipping with Android 4.2 have started going for sale in certain parts of the world, the Android 4.2 OTA for the GSM Galaxy Nexus is starting to roll out. Once one knowledgeable user gets their hands on the OTA, they make the direct download link available for everyone else, and that's what we've got now courtesy of the users over at XDA. According to the poster, this .zip can be flashed over either "yakju" or "takju" devices without issue. We've flashed it over takju, personally without issues.
Eager users with GSM models of the Galaxy Nexus on stock Android 4.1.2 can download and apply the update at their convenience, provided they have a few of the right tools and a little know-how. We have a great guide available that explains how to do just that right below, just substitute in the file names included in 4.2. There's also no shame in waiting for the OTA to hit your device.
Source: Google Direct Download; XDA