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How does a GMAC in GCM not nullify the utility of a seekable decrypt-able stream

I have been looking at the Galois/Counter Mode (GCM) for stream encryption. I understand the idea that it’s a stream cipher and so it can be encrypted one bit at a time by XORing with the key stream and how this is better than something like CBC because you can start anywhere in the middle ::Listen

I have been looking at the Galois/Counter Mode (GCM) for stream encryption. I understand the idea that it’s a stream cipher and so it can be encrypted one bit at a time by XORing with the key stream and how this is better than something like CBC because you can start anywhere in the middle of the stream without calculating previous blocks. But what I don’t understand is that in order for this to be useful you need to authenticate the GMAC, which as far as I can tell can only be calculated by knowing the entire stream from the start, as each block of the GMAC is derived from the previous one.

Could anyone explain how the requirement of the GMAC doesn’t nullify the usefulness of being able to decrypt a message starting from anywhere in the stream?