Maybe he wasn't aware of it? Just my speculation.
In any case, AES-GCM is natively supported by OpenVPN 2.4, and Streisand as it is will configure it for you as part of the NCP ciphers list, with a fallback to AES-CBC if the client is a bit older [0]
[0] https://github.com/jlund/streisand/blob/master/playbooks/rol...