The driver for this should be part of the Windows 7 OS. Just plug the USB cable into the computer with the MP3 player in USB mode. This is confirmed in the Windows 7 Compatibility Center.
The only driver on the GPX website: http://www.gpx.com/store/p/186-MW3836.aspx
is for Win98. See the Downloads section.
I suspect if the unit is not registering with the computer, you may have a port issue or cable issue. Try a different USB cable; it looks like a 4 pin mini-B to A USB cable. Make sure that the ports work on the computer and that they aren't disabled for security or power saviings. (The first is under Device Manager (Start > Control Panel > Hardware and Sound > Device Manager); the second is part of the advanced settings for the screen saver (right click on the Display where there are no running programs or icons and choose Personalize > Screen Saver > Change Power Settings > Change Plan Settings > Change Advanced Settings) Try a direct connection if using a USB hub.
I hope this helps.