For Redmine 3.x.x or Redmine 4.x.x.
Copy the plugin directory into the $REDMINE_ROOT/plugins directory. Please note that plugin's folder name should be "redmine_gitlab_adapter".
Install 'gitlab'
e.g. bundle install
(Re)Start Redmine.
Login redmine used redmine admin account.
Open top menu "Administration" -> "Settings" -> "Repositories" page
Enabled "Gitlab" SCM.
Apply this configure.
Login redmine used the project admin account.
Open this project "Settings" -> "Repositories" page.
Click "New reposiory".
Select "Gitlab" from SCM Pull Down Menu.
Paste <Gitlab Project URL>
into "URL".
Paste <Gitlab API Access Token>
into "API Token".
Click "Create" button.