Specify when you started noticing the issue (date and hour, as accurate as possible).
Provide a stepbystep description of what you're trying to do, and where is it failing.
Provide a screenshot of any error messages you're seeing.
Download the support tool from the page at https://github.com/google/gaps-support-tool/releases/tag/1.0.1 and run it. It will gather all the information required to diagnose issues with GAPS. Once it's done, attach the ZIP file it creates to the case.
If only one DC is affected, specify what is the name of the affected DC.
If the issue is affecting specific users, choose one affected user and provide an LDIF dump for that user. We can then look at the 'mail', 'pwdLastSet' and 'sAMAccountName' attributes for that user. This isn't necessary for installation issues.