I've just discovered the Assets export feature in Gravit - thank you!
Video here: https://www.facebook.com/GravitDesigner/videos/842669439224899/ (I've seen the YouTube videos but I don't use Facebook much, did not realise there were more videos on Facebook).
For other readers, if you mark something (object, group. slice) as exportable with the "Make exportable" button at the bottom of the Layers panel, you can use File menu, Export, choose Assets, and export every item individually.
This is great for creating a folder full of assets for other software (e.g. video editor), pieces for cut-out animation, or other purposes - all from a single project in one step!
In the layers panel you can also set export options for each item - a list of size variations, format (PNG/JPG/SVG), filename suffix.
Thank you Gravit developers, this is one of those desired features I did not realise was already present in Gravit.