Lurker de-lurking here...
I have a 2009 MacPro hacked to run High Sierra (4.1->5.1). Flashed AMD Radeon HD 7970 card and 2 calibrated Cinema Displays.
Ever since FCP 10.4.6 (or perhaps 10.4.4?) my FCPX viewer is displaying things with incorrect gamma. Basically the big classic 1.8/2.2 shift. Exported projects are displayed properly in Quicktime, etc.. I thought the days of Macs misinterpreting 1.8/2.2 were long gone......
The filmstrip previews in the file browser are correct gamma, except for the frame my cursor is hovering over - which exhibits the same display problem. I'm currently running Panasonic GH4 standard files (non-V-Log) with no special setting or treatment on them, but I've observed the problem with older FCPX projects I've brought out of archives to test, and these include some different footage.
I read somewhere that FCPX performs a gamma adjustment based on the system color profile and source footage. I've tried using different profiles, including those w/version 2 versus version 4 profiles, and even old archival Apple ones like HD 709-A, etc, to no avail.
Though the 7970 card is not on the approved list, it's Metal-capabable, though I know the OpenCL version is 1.x series, if that matters. I've had no other graphics issues anywhere except the FCPX viewer.
Is this just some new deep under-the-hood FCPX/High Sierra behavior that just doesn't work with my MP2009/7970 setup? Unfortunately I don't have another more modern card at the moment to swap out. I certainly don't want to add a temporary gamma adjustment to all the many chunks of my project.
Thanks for any wisdom you guys can offer... I may resort to installing Mohave on an external boot drive to see what happens.
Rather than relying on various GUIs, each of which may render/show video differently, have you tried taking a sample clip before importing into FCPX and after exporting and using a tool to compare the gamma?
At least you'd know if the actual video is being modified. Or, have you already checked this?
Most of the time I edit on a 2012 Mac Pro with a GTX Titan X GPU. Using macOS 10.13.6 and FCPX 10.4.6, I haven't noticed any issues... I do recall this issue many years ago, for me, with "classic" FCP.
Personally, I don't use my Eizo displays for any sort of definitive visual evaluation. I rely on a calibrated external monitor driven by a dedicated video I/O card.
As a quick check, I viewed a couple of video clips from my current project (HD ProRes422 and old DV-25 SD content). Everything looks the same everywhere in FCPX, as well as in QT Player, QT Player 7 Pro (7.6.6), Cineplay, and Switch. My computer/GUI monitors are HW self-calibrated once a week.
I have used "Gamma Shift Detector" by Digital Rebellion to examine files.