Sadly even in 2022, the modern branch of GZDoom still does not support borderless fullscreen window. It may appear it does to some users, as it does support windows 10+ fullscreen optimization, so alt tabbing will appear nearly identical on some configurations. But on a modern machine set to ultra low latency, on a GSync ultimate monitor, no, windows fullscreen optimizations not fully supported. You get no flicker alt tabbing out, but when you return, you do get a second or two of black screen flicker. Where as with true borderless fullscreen, it's 100% smooth. I tested this extensively on many many version of GZDoom. Only the Vintage branch has native support, so I left that in.
Very strange the older vintage branch is superior in this regard, but it is as it offers native support. In addition on my 2560x1440 setup, with Borderless fullscreen forced via SpecialK version 21.2.15.0 (newer versions crash the game) .. It still believes there is a border and thus forces it's internal resolution down to 2538 x1384 - this is displayed in the console upon launch. It can be temporary remedied by going into video mode and changing the resolution back to 2560x1440 and reapplying, which doesn't actually change the real resolution as the modern branch has no support for that.. But it than see it's currently functioning at that res and thus applies it to it's internal resolution scaler.
One functional modern workaround I could find is to use an above 100% scaling option. So 125%, with the border pixels loss I end up with 3172x1730 internal resolution upon launch. It's at least above 1440p so I let it stay at that.
Only other workaround I found was to use a very old version of GZDoom before they implemented this strange lack of real resolution support. 1.9.1 works well and launch's/stays at internally 1440p with Special K applied. Though it only has a 32-bit version, lacks console text scaling so the console is hard to read, tho all other UI elements scale well. And ofcourse lacks some features from later releases. Though another upside is it runs Brutal Doom ultra smoothly even with crazy high blood settings, while version past 3.0 perform worse, and tend to tank to around 40-60fps after too much blood decals have been applied, even an ultra high end system.