Hi and welcome to the forum (and to SUSE)!

What is the difference between SLE-15-SP1-Installer-DVD-x86_64-GM-DVD1.iso and SLE-15-SP1-Installer-DVD-x86_64-QU1-DVD1.iso
Can you share some information - maybe a link - to the latter iso? I'm not aware of "QU1" ISOs. "GM" means "golden master" and is the state of the product at release date. Prior to GM there are release candidates "RC" during development, but I have never read about "QU".