What is a SQL Server "GDR"?
A General Distribution Release (GDR) is simply a kind of hotfix, or patch that contains specific fixes, and is intended for installation on all SQL instances.  This is in contrast to a Quick Fix Engineering (QFE) hotfix, which is intended to be installed only if you experience a particular issue.

After a hotfix, it will also be included in the next Cumulative Update (CU) or Service Pack (SP).  But you can install the hotfix without taking all the changes included in the next CU/SP.

Going forward GDR hotfixes are only going to contain security fixes.

For SQL Server 2017+ a GDR is part of the Modern Servicing Model (MSM), which is replacing the older Incremental Servicing Model (ISM), which will continue to apply to SQL Server 2016 and before.  It's explained here.