I've never done this, so this is just me theorizing. Zip codes do not indicate a location by themselves. You have to have a database that have the zips and their corresponding GPS locations (many are for sale, a free one is available at http://www.census.gov/tiger/tms/gazetteer/zips.txt ). Then using basic algebra, you use the following formula

r^2 <= (x-h)^2 + (y-k)^2

r being your radius, x and y being your testing points and h and k being your zip codes location. If a real number is returned x and y are within your circular radius.

There are probably algorithms to ensure you are only checking ones in the right geographic area.

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