GLFP are different from SDFP because they can be made in advance. SDFP are strictly "same day" and can only be made at kiosks. GLFP are possible because the phantom MDE profiles have linked tickets. Those tickets won't be used to enter the park until some future time for a future trip. The tickets exist on the profile in order to allow the guest to book (or "generate") an extra set of FP+ in advance for a current trip. It's expensive to buy two sets of tickets for the same group of tickets, so not everyone will want to invest in GLFP. But you may end up saving money because the
ticket prices will inevitably go up before your 2017 or 2018 trip (at which point you might be ready to use those "generator" tickets for park admission).
When the SDFP+ loophole is eventually closed, GLFP will probably continue to work for a period (maybe a long period).