mnb It is not clear to me whether this needs to be done in inkscape or in Sozi
You cannot add graphic elements to your document in Sozi. This should be done in Inkscape.
mnb I tried and failed to do so.
Please explain what you tried and how it failed.
As far as I know, Inkscape already supports adding GIFs (choose "Import" from the File menu).
If the GIF is not animated when you play the presentation, then this is an issue with your web browser.