Which version of PHP Mailgun REST API Addon plugin requires?
Mailgun REST API Addon plugin requires one of the following PHP versions: 7.0, 7.1, 7.2, 7.3, 7.4, 8.0. The plugin might work with older PHP versions, but they are no longer tested or supported.
Which version of WordPress Mailgun REST API Addon plugin requires?
Mailgun REST API Addon plugin requires one of the following WordPress versions: 4.9, 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6. The plugin might work with older WordPress versions, but they are no longer tested or supported.
Which version of ClassicPress (WordPress fork) Mailgun REST API Addon plugin requires?
Mailgun REST API Addon plugin requires one of the following ClassicPress versions: 1.0, 1.1, 1.2.
How old is Mailgun REST API Addon plugin?
Mailgun REST API Addon plugin is 2 years, 2 months old.
When was Mailgun REST API Addon plugin first version released?
Mailgun REST API Addon plugin was first released on June 24, 2019.
Where can I find list of changes in all Mailgun REST API Addon plugin releases?
The detailed changelog for Mailgun REST API Addon plugin is available here: Mailgun REST API Addon Changelog.
Can I translate Mailgun REST API Addon to my language?

Yes. POT file is provided as a base for translation in the root folder for each plugin. Translation files should go into languages directory, and WordPress must be set to your language to load the file. The file name for translated and compiled MO file must be the same as POT file with the addition of language code: {addon-name}-{language-code}.mo.

You can check out this for translating plugins using POEdit program:
wp.dev4press.com/tutorials/translating-plugins-or-themes-using-poedit

Can I use Mailgun REST API Addon with GD Mail Queue Lite/Free version?

No.

The addons are made for the GD Mail Queue Pro, and there are no plans to have addons supported in Lite/Free version.