Prince Harry, 34, arrived with Peep Show actress Sophie Winkleman, who is the wife of Lady Gabriella's brother Lord Frederick Windsor.
Also missing from the festivities were Prince Charles and wife Camilla, as well as the Duke and Duchess of Cambridge.
Last week, Kensington Palace announced Prince William would attend the 2019 FA Cup Final in London as part of his Presidency of the organisation.
The sporting event clashed with Lady Gabriella's wedding and 9Honey report it's likely Kate stayed home with the couple's young children, Prince George, Princess Charlotte and Prince Louis.
Despite her absence, Kate's family were guests at the wedding.
Her parents, Carole and Michael Middleton, attended with younger sister Pippa and her husband James Matthews.
The Duchess of Cambridge's brother James was also present with his French girlfriend, Alizee Thevenet.
Lady Gabriella, a public relations director who BBC report goes by the name Ella, wore a lace bridal gown by Italian designer Luisa Beccaria.
Her husband Thomas, 40, is a Bristol University graduate who works in finance.