GPS is a simple addon for displaying the location of guild members on the guild map.
In order for a guild member to be displayed, they must have this or another LibGuildPositions-compatible addon installed (such as Cartographer's GuildPositions module).
Credit to Ckknight, from whom I shamelessly stole the artwork used for map blips.
If you're interested in consistency between the map tooltips, you may want to check out my addon BlipTips. It reformats the default party/raid tooltips to match the guild ones provided by this addon.
Note: This addon works with Mapster.