Yes. See posts #505 - #509 in this thread where possible guest list for Lady Gabriella is addressed, as well as references to Lord Freddie's wedding, which it was explained, took place at a time of the year when most of the royal family are away (e.g., the Queen takes her sojourn to Balmoral).
Let’s not forget that Edward and Sophie were mArried at St George’s in Windsor.
I don't know if it's been mentioned but Prince Charles and Camilla had their blessing ceremony at St. George's Chapel (at which the Queen attended, since she purposely was not present at their civil wedding ceremony).
St. George's is an absolutely wondrous venue, and I'm glad that Harry & Meghan chose the Windsor setting over Westminster Abbey. And that Eugenie had the wedding of her dreams there in October. As someone mentioned earlier, St. George's Chapel and the Windsor setting have a great deal of family historical significance and meaning. I'm not surprised that Lady Gabriella desires for her wedding to take place there too.
It will be interesting to see who will be on the guest list among the immediate royals. I would imagine that there will not be enough public interest nor a necessity seen for the wedding to be televised. Back in the 1960s, Lady Gabriella's Uncle Edward's (Duke of Kent's) wedding to Katharine Worsley in her home town, was televised (or if not, it was at least filmed and made available for newsreels). It was perfectly charming, and views are available on Youtube. Diana PofW's sister Jane was a bridesmaid, and her father Earl Spencer, was also in attendance.
For those interested, head over to the Royal Wedding forum to check out the thread on recent books about royal weddings, historic and contemporary.
... And as Mirabel pointed out, the highest tier of Royals have a state or semi-state wedding at Westminster Abbey (or St Paul's).
I'd heard that the Queen preferred for Prince Harry to wed at Westminster Abbey and it was offered. Therefore, it was possible for H&M to choose that grander setting. But as we know, Harry isn't into all the heavy duty pomp and circumstance. He preferred the more intimate family setting of Windsor for a number of reasons that are surely similar to Lady Gabriella's, i.e., fond memories of growing up around the Windsor environs.