Is there a way to get the full list of GPL-licensed apps in the Android Market via a URL or any other method?
I don't think that there's any standard way in the Market for developers to declare what license their app is developed under.
A normal text search of the Market for "GPL" comes up with hundreds of apps that have specifically put the word GPL in their title or description.
Other than that an awful lot of the Android GPL developers seem to host their code on Google Code, everything there is open source and there are ways that you can search that by license type, that might well be the closest that you can get (though you'll obviously miss out on developers who host their code elsewhere).
I don't know the answer to your question, but the closest thing I've found is: http://fdroid.org/