There are three types of grids available from the GPW site with different units for each: population counts (persons), population densities (persons per square km), and areas (square kilometers). The range is very large for the population counts and densities as population is “spread” over grid cells, resulting in surfaces that vary from very high densities in urban areas to extremely low densities in large rural areas where less than a person per grid cell is common.
The tail part of the Aleutian Islands in Alaska crosses the date line into the eastern hemisphere. As a result of the data structure which splits the world at the date line, the grid for the United States spans the world east to west. There may be additional cells outside of the date line at the edge for the United States, these can be safely ignored.