I asked a question and received quite good answers. I read all answers and other sources about my question.
I concluded that if I write my code and distribute it to my clients for a free under my own license and ask them to download F3 separately at their own, my code won't be covered under GPL license anymore as I am not providing the GPL code with my distribution.
For example, people are selling WordPress themes and plugins which are not covered by GPL but WordPress is licensed under GPL. Am I right?