composer require 'drupal/gin_lb:^1.0@RC'
only loads Installing drupal/gin_lb (1.0.0-rc5)
composer require 'drupal/gin_lb:1.0.x-dev@dev'
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.
Problem 1
- drupal/gin_lb dev-1.0.x requires drupal/gin ^3.0-rc11 -> found drupal/gin[dev-3.x, 3.0.0-rc11, 3.0.0-rc12, 3.0.0-rc13, 3.x-dev (alias of dev-3.x)] but it does not match your minimum-stability.
- drupal/gin_lb 1.0.x-dev is an alias of drupal/gin_lb dev-1.0.x and thus requires it to be installed too.
- Root composer.json requires drupal/gin_lb 1.0.x-dev@dev -> satisfiable by drupal/gin_lb[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.
Closed: works as designed
1.0
Code
Root composer.json requires drupal/gin_lb 1.0.x-dev@dev
Update your constraints to use non-dev, like ^1@rc
You could composer remove drupal/gin_lb, then composer require drupal/gin_lb to make it simple.