Alright, you've unlocked your bootloader and you're ready to tear it up with custom roms and other assorted mods. But wait, this is probably a good time to stop. No, I'm not messing with you. Justin Case and I only just acquired root on the G Watch on July 4th, and there's very little that can be done with it at this point. Additionally, there are no superuser managers (eg. Chainfire's SuperSU or Koush's Superuser) available to prevent an app from abusing root for malicious purposes. This situation will surely change over a matter of days, or even hours, but that's where we're at for the moment. I'm not suggesting that people avoid rooting, only that they wait until it can be done a little more safely.
If you're one of the people that wants to get started on building some root apps, or you just need to be on the bleeding edge, you're in the right place. So far, we only have the process worked out for the LG G Watch, but the Samsung Gear Live should be ready fairly soon.
Now, reboot into the bootloader:
adb reboot bootloader
And boot the G Watch using the modified boot image. Warning: Do not flash the boot image, just boot from it!
fastboot boot LGGW-rootboot.img
The watch will play the boot animation for about 2 seconds, then automatically reboot. This time it will boot normally. There will be no outwardly obvious signs it has been rooted, but you can check it by opening a shell over adb and running su.
If your prompt turns from shell@dory to root@dory, then you've got root on your G Watch. Congratulations!