When I launch gMTP for browsing the files in Galaxy Nexus I get the following error message:
Failed to get storage parameters from the device - need to disconnect.
I have already tried the answers given in the following question:
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If you just want to be able to browse files on your Android phone in 12.04, you can just use the suggested PPA in this good article
I personally find it slightly unstable compared to ubuntu 13.04's support, but may be I am nitpicking and overall it works just fine :)
The most reliable method for me has been to use go-mtpfs. It requires manual mounting/unmounting (via terminal - I did a little mini script to make it easier to remember).
Also, since you are on a Galaxy Nexus, I do believe the latest cyanogen mod roms have the ability to mount as mass storage OR MTP/PTP. If you're rooted, it may be worth checking out as well.