Remove key from an encrypted file?

[Benjamin Bressman]

If I use GnuPG to encrypt a file with multiple keys is it possible to
remove one of those keys at a later date?

Let's say I encrypt sensitive information so that three users could
decrypt it, but one of those users leaves the organization at some
point. Could I just remove that key's access to the file, or would I
need to decrypt the file and then re-encrypt it with only the desired keys?

I'm assuming the file encryption is symmetric using a "random" key, and
then that "random" key is encrypted asymmetrically once for each of the
multiple keys, but let me know if that's not the case.