Out of the box, Gio applications throw an error in VirtualBox Virtual Machines:
no support for OpenGL ES 3 nor EXT_sRGB
I've read in one of the comments that there are plans to provide a sort CPU-only fallback in the future, or something on those lines. But I couldn't find a ticket so I've created this one.
I was able to work around the error by following these three steps:
VBoxSVGA. This can be done in the UI under "Machine » Settings... » Display » Screen" or using the CLI:
VBoxManage list vms # to get the VM's UUID VBoxManage modifyvm «UUID» --graphicscontroller=vboxsvga
Tested on a openSUSE host with a Wayland+Gnome (Debian) guest VM and an X+KDE and X+LXQt (openSUSE) guest VM; though, I'm not sure if the proprietary Extension Pack is required to get this working.