As Junxiao said, it is dangerous to imply that IV is not an important parameter when using GCM. When reusing the IV in GCM, if an attacker captures a few encrypted messages, since basically GCM is an xor of the plaintext and the keystream (and the keystream is the same when using the same iv), then it is trivial to implement an attack that gets the keystream and unencrypts all the captured messages. It is basically the same attack that you would use for reused keys in One Time Pad. I highly encourage you to edit that part, otherwise the article is very useful, thanks.
As Junxiao said, it is dangerous to imply that IV is not an important parameter when using GCM. When reusing the IV in GCM, if an attacker captures a few encrypted messages, since basically GCM is an xor of the plaintext and the keystream (and the keystream is the same when using the same iv), then it is trivial to implement an attack that gets the keystream and unencrypts all the captured messages. It is basically the same attack that you would use for reused keys in One Time Pad. I highly encourage you to edit that part, otherwise the article is very useful, thanks.
Thanks a lot for the feedback! I'm changing the tutorial's approach to this.
Edit: done