You are incredibly naive if you think that's how these business work. He is not going to be out there blasting Disney for using his script that he was paid for and submitted. In fact, I would wager that Gunn has already decided to be the bigger person here and accept that Disney did what they felt they had to do. He will be in full support of his old crew in making G3. Doing otherwise is Tranking yourself. You don't burn bridges like that if you want to continue working in Hollywood, EVEN IF you were burned yourself.
We talking the same Gunn that publicly insinuated that Jared Leto is a pedophile? Dude is not known for being quiet, in fact that's why he's in trouble. And, as I said, he doesn't have to do much. The second it's revealed that they are still using Gunn's script it's going to cause another huge stinkwell on the internet that will eventually demand that Gunn respond with his thoughts. There will be plenty of tweets and articles asking how Disney can profit off of the work of someone whose character they believe is not fit for Disney. Now, maybe they can weather that coming PR nightmare, but the smarter move would just scrap his script and get their internal team to do a quick rewrite.