The raster::getData('GADM', country='USA', level=1) now defaults to GADM 4.0. To force version 3.6:
From version 3.6 onwards, GID codes are persistent, making them more reliable for long-term projects than older versions. Resolution: Typically around 10m resolution. ISO 3166-1 alpha-3 code for a specific country to use in your scripts? Download GADM data (version 3.6) download gadm data version 36 work
GADM 3.6 works in WGS84, but for area calculation or European mapping, reproject: GID codes are persistent
print(gdf.crs) # EPSG:4326 print(gdf.head()) reproject: print(gdf.crs) # EPSG:4326 print(gdf.head())