If this is a public facing application thee are free terms of use that you could use to get started. If this is an internal application, asset tracking or large volume public facing application then you will need a license. Contact the licensing team
for details at
maplic@microsoft.com. As for overlaying GPX data on Bing Maps take a look at this module:
http://bingmapsv7modules.codeplex.com/wikipage?title=GPX%20Parser
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