Sure it's possible but GPS is all about time. Faking the time by a fraction of a second will break the whole protocol. 1min is enough to transport the signed packets across the world.
Sky basically if your only authentication scheme is time, and the protocol is all about getting the time, it is very hard to detect proxing from other locations because the bound of accepted times basically needs to be big enough that you can be spoofed a meaningful amount.
Sky basically if your only authentication scheme is time, and the protocol is all about getting the time, it is very hard to detect proxing from other locations because the bound of accepted times basically needs to be big enough that you can be spoofed a meaningful amount.