Hello
I found a GitHub issue that discusses the silent installation of the GLPI Agent. According to the discussion, it seems that the issue was resolved and closed on August 3, 2021. You may want to check out the discussion and see if the solution provided there can help you with your problem.
If you are still having trouble, you may want to consider opening a new issue on the GLPI Agent GitHub repository and asking for help from the developers or other users who may have encountered similar issues. They may be able to provide more specific guidance on how to achieve a silent installation of the GLPI Agent using a GPO in Windows Server 2019.