Drupal Licensing

I have a customer who wants to build a SaaS platform that they would like their customers to subscribe to and create and collaborate content, publish, launch email campaigns and track the campaigns. am wondering if we can take Drupal and build modules on top of it and host it as a SaaS service?

I understand the code that we build has to be under the Drupal GPL norms. But does the GPL support this kind of a business model?

I did go through the licensing forums and FAQs and think I understood that it can be done. But these legal stuff are always confusing to geeks like me and wanted to clarify.

Appreciate all responses. thanks.