To be clear:

If you are a *G with a US airline (UA, CO or US) you only get lounge access in the US on international itineraries. If you are *G with a non-US airline you always get lounge access even on purely US domestic itineraries.

And actually, IIRC, even as a UA/CO/US *G, you can use non-US Star Alliance partners' lounges (e.g. the LH lounge at IAD) on US domestic itineraries. You just can't use UA/CO/US lounges.