Does anyone have or know where I can find an quirectangular topographic map of Earth showing Antarctica and Greenland without ice?
You could use the ETOPO bedrock version to make such a map in GPlates, QGIS or another software.
How would I load their model into GPlates?
You import it through the file menu.
There are tutorials on working with rasters in GPlates:
I know how to import rasters. Let me be more specific in my question. Do I download and import their “grid-registered netCDF”, “grid-registered georeferenced tiff”, “cell-registered netCDF”, or “cell-registered georeferenced tiff”?
A reason for my confusion was that I assumed one could only import images as rasters, and these didn’t seem like images to me (though I’m probably wrong). I’m also confused because, if it’s already an image, then that’s what I want. In general I am very easily confused.
Yes, try any or all of these and see what suits you best.