EurKEY The European Keyboard Layout

Spread EurKEY, vote for this answer this answer on StackOverflow. Or consider making a donation. You can also migrate the layout to other Linux distributions. In case you had, please let me know so I can make these packages available to public.

First, the AltGr key is equal to the right Alt key, so all key mappings are equally available with a normal right Alt key. It is however also possible to make use of another key, if AltGr is not easy to reach (or defect).

Linux

There are various answers to this question online. Just search for something like "xkb remap right alt key to caps lock".

Windows

I created a registry file that maps the Insert key to AltGr. You can have a look at that, but know that making manual changes to the registry may damage your Windows installation. If you know exactly what you are doing, and you know how to remove registry entries, you can try out this mapping. Here's to acitvate it: InsertToAltGr.reg, and here's to remove it: RemoveKeyMappings.reg. Again, only run it after having verified the registry files do what you want.
An alternative apporach for Windows would be https://www.autohotkey.com.

Uninstalling EurKEY is equal to any other procedure of uninstalling a program. If you experience problems (because the layout is still in use), please check this link: http://superuser.com/a/171162/384610 (it's for Windows XP but works for Windows 10 as well).

Windows loads layout DLL files early and keeps them in memory (probably for performance reasons) – even after uninstalling a layout. So replacing one version of EurKEY with another requires three steps:
1. Uninstall current version
2. Install new version
3. Reboot

Make sure the key isn't configured as hot-key in your operating system. KeePass 2 uses ALT_GR + A for 'Global auto-type'. See https://superuser.com/a/553138/384610 for how to disable this hot-key.

Apparently it's not possible. The closest workaround is described on https://superuser.com/q/592970/384610. If you have a better solution, please let me know.

Here's a small guidance I received from someone having problem with the installation and finally managed it:
1. Windows Settings > Time and Language > Region & Language
2. In "Language | Preferred Language", click on any Language, then magically a button "Option" appears.
3. Keyboards > Add a keyboard > Add EurKey

Gnome requires additional layouts to be activated with this command:

gsettings set org.gnome.desktop.input-sources show-all-sources true

See https://www.reddit.com/r/gnome/comments/7jqvl9/eurkey_layout_missing_from_the_settings.

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For LXQt and other window managers, there are two options.
For a per-user configuration, add or change the keyboard definition in ~/.config/lxqt/session.conf:

 [keyboard]
 layout=eu 

For a system-wide configuration, edit /etc/X11/xorg.conf.d/00-keyboard.conf and change the value of XkbLayout:

 Option "XkbLayout" "eu" 

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More infos for other distributions can be found here:
https://askubuntu.com/questions/482678/how-to-add-a-new-keyboard-layout-custom-keyboard-layout-definition/483026#483026.