Select create a new VM by selecting the
.vdifile created by VirtualBox. Gnome Boxes will copy and convert this file into it’s own format and import the VM. (The .vdi file is not required after the Gnome Boxes guest completed imported and can be deleted). You can change properties of the "box" such as RAM size, CPU count and storage if needed.
Start the existing VM that got imported, and uninstalled the VirtualBox Guest additions from inside the VM. Install the Spice Windows Guest Tools and Spice WebDav daemon to get automatic screen sizing, shared folders and other helpful features.
One great advantage of Gnome Boxers over VirtualBox iand its use of the extension pack, which under an EULA license forbids commercial use (if you need to get USB3 support for example). Gnome Boxes instead relies on QEMU, which is completely free software.