"g" — The data are stored in geographic coordinates of decimal degrees based on the World Geodetic System spheroid of 1984 (WGS84).The geographic coordinate system is designated by the letter “g” at the end of the naming convention.
“a” — National-level population estimates from the United Nations have been used to adjust the population totals for each country. The difference between the UN estimate and the source data estimate of total population for each country was used to calculate an adjustment factor. This factor was applied to the grids at the national level. Both adjusted and unadjusted grids are available. The "a" means it is adjusted to match UN totals.
A description of the file names should also be available in the readme file that comes with each file downloaded. For a more detailed description of GPW methodologies, please see the GPW v2 working paper, “Transforming Population Data for Interdisciplinary Usages: From census to grid.” at http://sedac.ciesin.columbia.edu/downloads/docs/gpw-v3/gpwdocumentation.pdf