For Windows, you'll need Microsoft Keyboard Layout Creator 1.4 which can be used to edit and save de_dmg.klc and also to create installer binaries from the the files inside of windows/.
For Linux, the Keyboard Layout Editor can be used. It doesn't offer the same convenience as Microsoft's offering for Windows, so the result will have to be saved into the de file and everything extraneous be removed manually; also mind the files in linux/rules/.
As noted in the README, de_dmg is an Xorg keyboard layout only. Given that at least standard Fedora Workstation seems to be sticking with Wayland, a port of de_dmg to Wayland would be a great thing to have! Check out https://brokkr.net/2019/01/11/customize-your-keyboard-layout-and-have-it-work-under-wayland/ and https://askubuntu.com/questions/991742/modifying-keyboard-layout-in-wayland maybe.https://www.aoi.uzh.ch/dam/jcr:14946514-bef9-4f68-9e8e-5521d5a58d0e/Trscr.keylayout
I can't think of anything that needs doing. If you would like to create a layout of your own, use Microsoft Keyboard Layout Creator and load the layout that your own should be based on, then make the desired changes, give it a name, and recompile.
For distributions other than the ones listed above, I'd be thankful to hear about the command line for installing the equivalent packages.
Unfortunately, due to the way Xorg-based Linux distributions store keyboard layouts, it's not a simple task to create distribution packages. For Debian, Stephen Kitt over at StackExchange outlines a way that would work. Help with implementing that idea is most welcome!
The only way to have de_dmg be available out-of-the-box, without having to install it, would be to get the dmg partial included "upstream", that is, get it accepted into the XKB-Config project. Whoever feels inclined to working with them on their mailing list, you'd be doing a solid to every user!
It'd be quite easy to create a layout for macOS, but I've never owned and never will own a Mac. If anyone who reads this and does own one would like to contribute, I'd most happily accept a PR for that. According to this StackExchange answer, you can use a software called Ukulele, by the venerable SIL International, to make macOS keyboard layouts.
Aside from using something like Ukulele, macOS keyboard layouts are simple XML files. The University of Zurich has a layout available that provides the diacritical characters, but no pre-defined composites like the ones provided by de_dmg. It would be a nice starting point for a macOS-Layout as part of de_dmg.