The rest of the Middleton family also made the walk down the hill to the chapel, with Carole and Michael Middleton seen a few paces behind Pippa and her husband.
James Middleton was spotted walking hand-in-hand with his French girlfriend Alizee Thevenet.
The Queen is also in attendance, accompanied by members of the royal family including Prince Harry.
Her Majesty arrived a few minutes ahead of the bride with the Duke of Edinburgh, who was making a rare public appearance.
Philip appeared in good spirits and joked with the Dean of Windsor, the Right Reverend David Conner, who was waiting to greet the monarch and her consort.
The Queen wore a pink A-line coat, and a lilac and pink silk dress by Stewart Parvin, with a matching hat by Rachel Trevor-Morgan.
Lady Gabriella is the daughter of Prince and Princess Michael of Kent.
Another famous face was Downton Abbey creator Julian Fellowes, who was in the first wave of arrivals for the ceremony taking place at noon at St George’s Chapel, Windsor Castle.