The Duke & Duchess of Gloucester are possibly the least well known of the working royals, but I don't think they are unknown, especially by those attending a Garden Party at Buckingham Palace.
Yougov has the Duke's "fame" (the percentage of people have who heard of a person) as 43% - so while it doesn't compare to the 100% of people who have heard of William of 98% who have heard of Charles it shows he isn't unknown.
The most famous royalty in the UK according to YouGov Ratings. Fame is based on millions of responses from the British public and YouGov's innovative survey methodology.
yougov.co.uk
For comparison in the fame ranking of public figures he sits at 149th position out of 406.
He sits above Lisa Nandy (UK Cabinet minister, Secretary of State for Culture, Media, and Sport) at 42%, Narendra Modi Prime Minister of India at 42% fame, Aung San Suu Kyi at 41% fame, Bridget Phillipson (UK Cabinet Minister, Secretary of State for Education) at 36% fame. But below the likes of William Prince of Wales, King Charles, Elon Musk, Boris Johnson, Bill Gates, Theresa May (former UK Prime Minister) who all rank above 95% fame.