There are two global elevation data sets that I know of, both are at 30 arc-seconds (0.5 arc-minutes) in resolution (GPW is 2.5 arc-minutes). You can get them on the web at:

GTOPO30
http://edcdaac.usgs.gov/gtopo30/gtopo30.html

GLOBE
http://www.ngdc.noaa.gov/seg/topo/globe.shtml

To get these to match to the GPW grid you just need to resample them by a factor of 5, I’d recommend either cubic convolution or bilinear as the interpolation method.

There is also detailed elevation data for different parts of the world from NASA’s SRTM but it is too much detail in comparison to our GPW data.