For the purposes of this thread and being a guest at PPH: PPH is just across the street from GCH with a pedestrian light crossing that if you could always use the GCH shortcut would always be the fastest/easiest way to get to the parks. To use the shortcut you would cross the street towards GCH, enter through the main lobby and then navigate from their lobby to either of the two paths to the parks shown in maps in the thread below.
However, since the GCH is outside of the security bubble that encompasses DTD and the two parks, and the space they have for security at the GCH isn’t all that big, if they expect a flux of crowds to try to access GCH they will limit who can enter the GCH at all, especially from the direction of the PPH to not have those chokepoints back-up and give the guests paying the deluxe prices to stay (and dine at the GCH table service restaurants) priority. So it may be that they require you to have business specific to either being a guest/dining with an ADR to enter the lobby of GCH from that main lobby entrance right across the street from PPH. If they limiting the entry, then to get to the parks, PPH guests have to walk towards the Disneyland Hotel which is a total back-track away from the parks, do security at the entry of DTD (with the DLH hotel guests but there is vast amounts of space here for security and lanes are always proceeding clear) to then walk the the entire length of DTD (which is REALLY small compared to WDW - it can be done in under 10 min at a speed walk and a more normal pace is 15 or so min.- so not bad at all). But because of the back-track is remarkably longer than the shortcut through the GCH. And if you’re still confused, go ahead and open the maps in the thread and ask away on any questions you have on it and we can add clarity to the GCH entrances.