As far as I know map/image overlays are not possible in Gramps right now. I’m currently working on a heatmap addon and the package I’m using there also has options for image overlays which could be used to create that feature in a new addon.
I looked at some of the options of the Georeferencer website.
An option exists to output map tiles. Gramps can use such tiles when it plots Events. (As long as the tiles are 256x256 pixels and follow the naming convention.)
This would not be the ideal solution. It does not allow you to use all the interactive features of the map service. But at least you can access one of those period maps which has been re-shaped to a modern GPS grid.