I looked at the nut and the top of the string is below the top of the groove (which is usually an indication - along with the buzzing - that it's either cut or worn too deep.)
It's not horrible... just noticeable. So, there's no huge hurry.
- I could try to replace the nut myself, but I'm shaky on it given that Gibson sandwiches the nut between the fretboard and the headstock overlay, almost requiring a hacksaw to remove the original. I'm sold on unbleached bone, by the way, so if it's replaced, it'll likely be with that material.
- I could send it to a tech - more expense but peace of mind.
- I could fill with CA and baking soda and re-notch. I'm not sure if this is a long term solution, so if anyone has any information, I'd appreciate it.
- I could also go with a slightly larger gauge string set for a while, which might not seat as deep in the nut. Not sure on that one.