If you distribute a modified version of Piwik, or software that includes Piwik as a module, or develop a website for a customer/client that integrates closely with Piwik, you must give access to the code source and publish all the code under the GPL.
However, it also states:
The data coming from the Piwik API belongs to you and is not covered by the GPL.
You can use Piwik as a service, and reuse, publish, display, modify, process the data coming from the Piwik API using the REST API web services call.
… so does that mean If I use Piwik Tracker, APIs and the database, but NOT its admin front-end, then GPL doesn’t apply to me? I hope thats a yes!