When you run Warcraft III Replays after installing W3GPC they don't get opened in Warcraft III immediately. The first step is, that W3GPC tries to verify the map path of the replay.
If this succeeds, Warcraft III gets called to open the original replay and view it.
If this fails, a special search routine is started in order to find the required map. After a successful search, W3GPC generates a temporal copy of the replay and adjusts the map path in it. Now Warcraft III opens the temporal replay with the corrected map path and should start viewing it without any problems.
If W3GPC couldn't find the required map, the map needs to get downloaded from the internet.