A bit of follow-up here. I still have my previous Android phone (on which I am able to make purchases) and happened to notice when I was in the accounts area of the Play Store that it says I'm connected to the US instance of Google Play.
(See motoG.jpg.)

However, when I navigate to that screen on my GrapheneOS phone, it says "Switch to the United States Play Store". (See grapheneOS.jpg.) There is no button or field to tap on, and tapping that text does nothing. So, something may be amiss with the sandboxed store? (At least, perhaps, for this particular installation.)

    8 months later

    mnifty90987 I don't think so. My
    sandboxed version says I'm connect to united states. Did you come across anything else?

      I stumble across this thread after running into the same issue. Could someone place a warning in a more prominent way that gift cards are incompatible with GOS? I was not expecting this.

        5 days later

        schweizer
        For what it’s worth, I had success in forcing Google support to redeem for me two Play cards on the same – pseudonymous – account, twice. As far as I know, my account has not been unflagged, though, so I’m still relying on communicating with them in order to redeem any new purchased Play card.

        The key is patience and diverse forms of legal threats. I can elaborate if needed, although it’s not very elaborate…

            schweizer
            Oh, I see.
            The worst I got, after having sent several times images of the card and the receipt, was ‘due to some inconsistencies with your account and the information that you provided, the gift card team isn’t able to help you redeem gift card’ and something along the lines ‘limitation and scope of support’ of the person answering me.
            That was after more than a month since my first request.