GADMTools is an R package to manipulate shapefiles from GADM and to make geo-statistical representations easily. It supports SpatialPolygonsDataFrame and sf (Simple Features) formats from GADM database.

GIT: https://github.com/Epiconcept-Paris/GADMTools

GADM is a spatial database of the world's adminstrative boundaries for use in GIS and similar software. Administrative areas in this database are countries and lower level subdivisions such as provinces, departments, cantons, etc. Please, visit gadm.org

NB: https://gadm.org is not the creator nor the maintener nor responsable of GADMTools

With GADMTools, a wrapper for GADM shapefiles, you can easily manipulate, assemble, create subsets of these objects and plot statistics on the final maps.