Your requirements could not be resolved to an installable set of packages.
Problem 1
- drupal/gmessage dev-1.0.x requires drupal/group ^1.0 || ^2.0 -> found drupal/group[dev-1.x, dev-2.0.x, 1.0.0-alpha1, ..., 1.x-dev (alias of dev-1.x), 2.0.0-alpha1, ..., 2.0.x-dev (alias of dev-2.0.x)] but it conflicts with your root composer.json require (^3.0).
- drupal/gmessage 1.0.x-dev is an alias of drupal/gmessage dev-1.0.x and thus requires it to be installed too.
- Root composer.json requires drupal/gmessage 1.0.x-dev@dev -> satisfiable by drupal/gmessage[1.0.x-dev (alias of dev-1.0.x)].
Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.
Installation failed, reverting ./composer.json and ./composer.lock to their original content.
I probably will get to this eventually but I'm not currently working on any sites using group 3 so it may be a while.
That said the module is pretty simple and I don't think it touches any of the parts that actually differ between the 2.0.x and 3.0.x version. It likely will work if you install it. If you can test it locally and confirm I can update the composer.json to indicate group 3.x support as well.