It does on the entire KDE desktop, and any Qt 5 app. Why do you keep repeating this?
Because this is only true if you use an OTF, which almost no distro ships with by default, so no QT doesn't do gamma correction by default on Linux. And as I said even if it did, the most major applications on Linux still don't do it, so have fun with half your apps looking decent and the other half looking like crap.
Oh, and by the way, on my 2560 x 1600 pixel Macbook
Mine doesn't do that nor any MBP I used so far, not saying you're lying or anything though, just saying it's anecdotal