In an article on my 4x great grandfather Gaspard Weiss (1739-1815) in Sortir à Mulhouse (which appears to be a newsletter on some sort of cultural program for Mulhouse - its in French so I'm not completely sure!), number 36, a reference is made to him being one of the musicians who interpreted Amor Vincitore before King George III. I tried to find information on this a while back but was unsuccessful.
I've just come across a new reference to this in "Clarinet in the Classical Period" by Albert R. Rice (2008). Amor Vincitore was a cantata by Johann Christian Bach - youngest son of Johann Sebastian Bach. Rice writes that two movements of the piece have solo parts which were written for four virtuosos - Karl Weiss (flute), Johann Christian Fischer (oboe), Josef Beer (clarinet) and Georg Wenzel Ritter (bassoon). "Karl" was a German form of the French name "Gaspard", and there are other references to Gaspard Weiss as "Karl Weiss". Gaspard Weiss often worked with the oboist Fischer. Rice also says that in 1774 Amor Vincitore was performed in Carlisle House in London (on April 15), at a benefit concert for Fischer, and at a private performance for the royal family. Obviously if you were having your new cantata performed for the royal family you would make sure it was being performed by the musicians it was written for - hence we know that Weiss almost undoubtably did play for King George III.
I still have no definitive proof that Gaspard Weiss was the principal flautist for King George III as claimed by his son Charles Nicholas Weiss, in his letter to the music dictionary publisher John Sainsbury. Perhaps if the claim was an embellishment of the truth, this may have been the event Charles was referring to.