KitchM First off, if you purchased a gift card that is somehow tied to your name or account, you just lost your privacy/anonymity.
Pay cash at a random gas station or grocery store. Wait 30+ days to use it. It's basically untraceable. It won't stand up to a major NatSec investigation without some more steps but it will defeat anything less than that generally.
It's general good practice to buy a few such gift cards as the opportunity arises while traveling and just let them age in your safe for a while. Surveillance video tends not to be kept for more than 90 days at the absolute max unless a specific reason to save a specific clip exists.
Second, it spending the funds requires any sort of activation, beware.
Visa gift cards don't require any "activation" beyond the initial purchase.
How you do that defeats a lot of the use of Pixel/GrapheneOS in the first place. Why would anyone go to all the high expense of obtaining that package to break the purpose of it? The OP is assumed to desire an untraceable and ID-less service provider. Am I wrong in that?
GOS is used by many different people for many different reasons. As for "untraceable", that is always a matter of degrees and opposition.
At one extreme you have what the US NatSec establishment can do in pursuit of a priority target. Basically, the kind of targets who aren't concerned about courtrooms but B-2's with bunker busters, Delta pulling them from bed at 3am, or a Flying Ginsu shredding them in the back of their car one fine day.
At the other extreme you have your 15 years old stepson thinking you are cheating on his mom and trying to investigate.
As a practical matter, for 99.999%+ of threat models, Cape will provide either substantial risk reduction or even its outright elimination. It will do this without any real quality of life costs or required lifestyle changes.
If you are in the US and your threat model accepts having cellular service for your device at all then it is far and away the best choice. And if you want $20 a month off your bill, feel free to reach out via the SimpleX link in my profile.