Prince Michael and Princess Michael's daughter, 43, looked elegant in floral shirt dress and a wide-brimmed peach hat as she travelled in the carriage procession at the start of the Berkshire races.
It comes after Lady Gabriella made a low-key appearance at Trooping the Colour last Saturday, where she was pictured watching the flypast from inside Buckingham Palace, as the senior royals gathered on the balcony.
It has been a difficult year for the British writer following the unexpected passing of her husband and financier, Thomas Kingston, who was found dead at the age of 45 at his parents' home in the Cotswolds on 25 February.
The King and Queen are said to have been quietly supporting Lady Gabriella and she has also reportedly moved back into her parents' home at Kensington Palace.
Also joining the royals on the first day of Royal Ascot were Camilla's sister, Annabel Elliot and her nephew, Ben Elliot.
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First major outing
Lady Gabriella was seated in the second carriage with Princess Anne and her son, Peter Phillips, as well as John Warren - the King and Queen Elizabeth II's racing manager.
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Outfit details
Lady Gabriella looked beautiful in a pink and purple floral themed shirt dress, accessorising with gold and pearl drop earrings.
She wore her blonde locks loose under her peach-hued hat.
Meanwhile, Princess Anne recycled a red silk coat dress from Shibumi.
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The King and Queen
The King sported a grey morning suit with a sunny lemon waistcoat, while the Queen looked beautiful in a cobalt blue ensemble with a matching hat and a sapphire brooch.
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A warm welcome
The King and Queen wished racegoers the "best of luck" in chasing the "thrill of a Royal Ascot winner" as the famous meet began.
Writing jointly in the official programme, Charles and Camilla said: "It gave us immense pleasure and joy to witness Desert Hero's victory in the King George V Stakes last year, particularly as he was bred by Her late Majesty.
"We wish all the participants this week the very best of luck in seeking that unique thrill of a Royal Ascot winner.
"We also hope that those here at the racecourse and watching from home will enjoy five days of the finest racing that this sport has to offer."
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Full look
A full-length look at the King and Queen's race day attire as they stepped out of the carriage.
It was founded by Queen Anne in 1711 after she declared Ascot ideal for "horses to gallop at full stretch" and has today become a major social event as well as a sporting attraction.
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Sweet moment
Lady Gabriella was embraced by Zara Tindall with a big hug as she arrived at the enclosure.
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John Warren and Peter Phillips
Peter Phillips is a regular at Royal Ascot, and has recently confirmed his new romance with NHS nurse Harriet Sperling.
The pair were spotted packing on the PDA at the polo earlier this month, as they were joined by Harriet's daughter and Peter's daughters, Savannah and Isla.
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Annabel Elliot
Queen Camilla's sister, Annabel Elliot, looked elegant in navy as she shared a carriage with Ruth Wood, Countess of Halifax.
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Laura Lopes
Queen Camilla's daughter, Laura Lopes, joined the racegoers for the first day of Royal Ascot.
The art curator shares daughter Eliza and twins, Gus and Louis, with husband Harry Lopes.
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Tom Parker Bowles
Camilla's son and food critic, Tom Parker Bowles, also joined his mother and sister on day one.
Tom shares children, Lola and Freddy, with his ex-wife, Sara Buys.
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The Princess Royal's outfit
Princess Anne looked elegant as ever in her red silk patterned coat, teaming it with a mint green hat and navy gloves and bag.
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Mike and Zara Tindall
Zara always wows us with her race days looks and the first day of Royal Ascot was no exception! The equestrian looked stunning in a puff-sleeve yellow pebble print shirt dress by Laura Green.
She wore her blonde bob in an intricate updo with a boater-style hat by Sarah Cant and accessorised with her Sauvereign Aphrodite Clutch.
Meanwhile, former rugby star Mike looked dapper in a morning suit with a pastel tie to match his wife's ensemble.
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Princess Eugenie
It was a right royal turnout on Tuesday as Princess Eugenie and the Duchess of Gloucester joined their family members.
While Birgitte opted for an all-navy ensemble, Eugenie looked pretty in a sleeveless mint number with a boater-style hat, a floral hair corsage and beige Aquazzura heels.
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The Duchess of Gloucester
Eugenie and Birgitte enjoyed a good catch-up at the race meet.
The Duchess accessorised her outfit with a very practical and stylish raffia bag. We're all here for that!
It comes the day after she was invested as a Royal Lady on Garter Day at Windsor Castle.
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Trophy time
Queen Camilla beamed as she took to the stage to present The St James’s Palace Stakes to Jockey Sean Levey, rider of Rosallion.
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