It's unclear just how long it took for the idea for the website to gestate. Did he think of it right away, or did it take months for him to come up with the idea?
When something is gestating, it's literally in the womb, developing into whatever it will be when it's finally born. Other things besides living organisms can gestate, however. Frequently, gestate is used to refer to ideas and plans while they're in the development phase. The idea for a new way to market music to a generation of kids who expect it to be free was gestating for years before iTunes was launched. A gestation period is the time it takes for something to be born. Humans, for example, gestate for nine months.