The Apexis Gemcutter states that the only way to obtain the Gemcutter module: mastery is from the Brawlpub quartermaster. I can enter the pub (thanks to Wild Willy’s drop of the bloodied scroll) and can see Quackenbush’s wares, but I have never set foot in the Brawlpub and have no rank to purchase the Gemcutter module. Read Blizzard’s article that the Brawlpub has been deactivated as of Shadowlands. Seems it still is as I am standing in the Brawlpub now and nothing is happening with no way to start a fight, how am I supposed to obtain the module?
As it stands, you can’t. The only source is deactivated and Blizzard has neither reactivated it (usually as part of a season, and after tuning the content up to be level and gear appropriate) nor provided a new source for the module.
There is no way forward until blizzard provides one.
They have stated in two interviews this year they plan to bring Brawler’s Guild back, but there is no indication of when that may happen, if ever. And there is almost zero chance they put it on another source. However, anything in the recipe journal should be able to be obtained by players. It’s very annoying that you can’t.