I'm basing my understanding in two sentences, one from GPL FAQ and another from Drupal FAQ.
From GPL FAQ: If I know someone has a copy of a GPL-covered program, can I demand he give me a copy? No. The GPL gives him permission to make and redistribute copies of the program if he chooses to do so. He also has the right not to redistribute the program, if that is what he chooses.
From Drupal FAQ: 11: Do I have to give the code for my web site to anyone who visits it?
No. The GPL does not consider viewing a web site to count as "distributing", so you are not required to share the code running on your server.