Hi There, I'm using Gammadyne Mailer to send out our campaigns and am trying to use the post-send script to mark the username of each client but no avel

Never used this and I don't see any detail docs, but from this page https://www.gammadyne.com/gm-personalization.htm it looks like you have it right with enclosing the field name with double brackets. What you probably need to do is make sure the field name is spelled correctly and using the same capitalization. Also check for any odd spaces or hard to read characters like a period that you missed.

On another note, you are probably going to be better off using a 3rd party service like MailChimp or ConstantContact to send your email marketing. Sending from your own server or even your personal computer is probably going to have low delivery rates.  Good luck!

By not using a third party as Scott suggests, you also might get your IP blocked as a spammer. That's never good, either.