How can I download one area’s whole raw gps data?

Raw GPS data for a limited size area can be downloaded through the OSM API. You would usually do that using one of the existing editors like Merkaartor or JOSM, and you can then save the data to a file. There are some limits to this form of downloading, most notably a size limitation of 0.25 degrees squared.

Raw GPS data is not currently distributed any other way. The cloudmade.com web site mentioned in another answer does not carry raw GPS data.

There are plans to offer a file nicknamed the "GPX planet" for download which would then contain all publicly available GPX data but that's not there yet.