There are multiple successful reports of people getting lounge access by showing a physical *G card while still accruing to UA. (You do have to be careful that they don't change your accrual to the other airline, but if they do, you should be able to change it back to UA in the app).

I don't love being able to mix and match the two programs like this, but (a) *A hasn't yet updated the rules to forbid this and (b) it seems a lot less objectionable to take lounge access under debatable circumstances than it does to take a CPU.

So if this is an acceptable compromise -- credit to UA since you're using UA benefits, but use the *G card for lounge access -- that's probably the way to go.