I use this. It's from a user named Electron.
http://www.wolfgang-rolke.de/gbxdump/gbxconv.zip
You can find the original forum post here:
http://www.tm-forum.com/viewtopic.php?p=182880&sid=f9be60ce2723bbccae3439b42b9f29a1#p182880
It was originally made for TM1 but seems to work fine for TM2 replays.
You'll have to rename the resulting files back to *.Map.Gbx though, because otherwise it gives you a *.Challenge.Gbx
The only thing is you won't have the screenshot/thumbnail on the map file. So what I do is then open the map in the editor and resave the map without driving it again. Then the thumbnail is created once more.
@Eternity, that's an interesting solution. Does that allow you to open the maps in the editor? I've never tried it.