ext-gmp installed but composer does not recognize it

Not all extensions are installed in the same way, if you see the second example, after installing via apt-get, you might need to enable it via pecl or docker-php-ext-enable, so this might be the case for your extension (see this for example https://stackoverflow.com/a/64445115).

See the redis module for example: https://git.drupalcode.org/project/redis/-/blob/8.x-1.x/.gitlab-ci.yml?r...

Documentation changes are more than welcome, as the docs live in the code of this repo too.

Thx @fjgarlin for acknowledging the problem is a problem ;-).
Still struggling with it, and opened a docs improvement issue.

Do you have an open issue in your module where we can maybe investigate / try things?

I burnt some hours to advance from the blind to the one-eyed, but not i got it.

Automatically closed - issue fixed for 2 weeks with no activity.