Click 'open' and see what list of filetypes can be opened. Chances are it could be that you downloaded the older version which doesn't support newer file types, only files such as .GCFs.
I assume you're trying to open a bsp, GCFScape does not support bsps that have been repacked, and you'll have to unrepack them first before trying to pull anything from them.
Vasya means that if the map VMF is available for download, you can probably also obtain the props in a download like it. If not, perhaps just ask the map creator where the props are from, etc