All leather goods, in fact...all good, can take some time to come in, based on production. Also, whether the particular store has placed an order in for that particular item already. Sometimes, you can make requests or your SA may offer to source items for you from other stores (less so now during Covid). But, these services are still done.
Although the GP bags (just like Picotin, Evelyn, Lindy...etc.) are not "Quota bags" per se, they are still leather goods that take the ateliers time to produce. Meaning, they will come in during different times during the year, month, weeks and so on and so forth.
In your case, what may have happened is the particular colour for the GP is not available in the store (i.e., present in the store) for sale to you readily. While it is not a quota, they just don't have the bag in stock. Your SA has already luckily notified you that more are expected to come, so you should be notified by your SA once they come in and are available for purchase (in the combo that you'd like).
For example, last year I wanted a picotin (small size, neutral colour combo w/ ghw), and luckily for me, there just happened to be a shipment of picotins. I got to pick whichever one I wanted. I just wanted neutral colours, so my SA put the neutral ones aside for me to choose when I booked my appt to go into the store. No wait time, whatsoever.