If you search hard enough, here and on netkas' forum, you could find few apps that were designed for flashing (usually ATI cards) on OS X.
I'll attach old Zeus app that i have in some random app folder, but i've never used it so i really don't know if it works or not. Don't blame me if something goes wrong.
Saying all that, it is really easy installing Windows on small partition, or spare HDD and boot into it just in case of emergency (to flash GPU or check it's temperatures under load). That way you won't loose hours trying to find app that can flash your GPU on OS X.
Once again, search is your friend
https://forums.macrumors.com/threads/release-ati-flashing-tool.811768/
https://forums.macrumors.com/threads/release-zeus-flashing-tool.877437/