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:

1. Install the "Oracle VM VirtualBox Extension Pack" in the host system.
2. Install the "VirtualBox Guest Additions" in the guest system.
3. Change the VM's "Graphics Controller" from the default VMSVGA to 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.