Specifying GPG Decryption Key

I'm working on a digital forensics project involving data recovery. Suppose I have the encryption key (not the passphrase, the actual key) for a file encrypted with GPG symmetric encryption and the encrypted file. How could I go about decrypting the file? Is there a way to specify the use of a specific key for GPG decryption instead of a passphrase?

I've considered just trying to decrypt the file content with Python, but GPG uses its own variant of CFB …

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