The range map is a polygon, while atlases are essentially a visualization of checklists at the country, state, and county levels. Some parts of the tree (like mammals) have gotten range maps from outside sources; most plants don’t have maps (see conversation above).
Here’s a mammal with both a range map (the pink polygon) and an atlas (the green political boundaries).
Depending on whether your target plants have good atlases and already have out of atlas observations, you may be able to get somewhere with those. If your species already has out of atlas observations, you can follow the link on the atlas page to view out of atlas observations – this just constructs a query for not in all the atlas places.
For example, this is the first plant I saw in the list of atlases with out of atlas observations. Clicking on View sends you to an Observations query that only has the out of atlas observations – they can be downloaded in the normal fashion by clicking Filters -> Download.
If you want to use range maps instead of atlases, I would imagine you can upload those polygons as new place boundaries and then search not in that place for out of range map observations.