You can duel license your commercial product. Only the PHP code that interacts with WordPress is required to be GPL.
If you really want an extensive GPL lesson you can peruse the Thesis GPL Saga files:
- WordPress, GPL, Thesis and Speeding Tickets
- Syn-Thesis 1 And Chris Pearson
- The Great Thesis Vs WordPress Debate Rages On
- The GPL is Marxist
- #thesiswp
- The Thesis Controversy
- WordPress Could Sue Popular Theme Developer Over License Dispute
- WordPress/Thesis Conflict Highlights GPL Nuances
- Thesis The Antithesis of WordPress…sort of
- On WordPress, Thesis, and profitable GPL Software
- WordPress And Thesis Go To Battle
- The Coming Thesis Vs. Automattic Battle
- WordPress vs Thesis Theme by Cory Watilo
- Thesis Pissing in the WordPress Well?
- Analysis of GPL’ed Code in Thesis
- The “Real” Story – WordPress vs Thesis Theme
- The WordPress vs Thesis GPL Debate
- WordPress vs Thesis Can WP Themes be Non-GPL?
- How Infectious is the GPL? Battle of Words Between WordPress and Thesis
- Thesis + GPL + WordPress = Pain
- WordPress, Thesis, GPL, and Premium Themes
- WordPress, the GPL, and Thesis
- Thesis and the GPL
- WordPress Matt Mullenweg & Thesis Chris Pearson GPL Controversy
- WordPress, Thesis, and GPL Heartache
- Thesis vs WordPress: GPL License Debate
- Another Take on the #thesiswp Drama
- Why the GPL Doesn’t Apply to Premium WordPress Themes
- @personified’s Ethos of Creative Entrepreneurship vs the Hive
- Why WordPress Themes are Derivative of WordPress
I’m not a lawyer and this should not be considered legal advice.