You need to decide whether you are using UEFI booting or traditional MBR booting.
It seems that your disk was partitioned for MBR booting. So that almost certainly means that Windows 7 was also installed for MBR booting.
Option 1: Reinstall Windows for UEFI booting. As part of this install, the disk will be repartitioned with GPT partitioning. Then install opensuse and you should not run into those problems.
Option 2: Boot the opensuse install media in MBR mode instead of UEFI mode. That should be possible, but perhaps it is easier if you disable UEFI in the BIOS before you boot the install media. Then it should stop insisting that the disk needs to change to GPT.
From your description, “option 2” looks like the easier way.