First, here is an engraving of Lord George (and a woman he was associated with), which appeared in Town & Country, June 1786. Although it is not known for sure why he was nicknamed Lord Crop, it has been speculated that it was because he was circumcised. However, if so, then it becomes clear that it was widely known that he had become Jewish by 1786, while for other reasons the date when he converted is not clear.
The gossip associated with the images is directly below:
Here's an account of what happened when he was apprehended (emph. mine):
“When the officers waited upon him, he did not deny himself, but told them he was a Jew, and whatever might happen, should continue one; and when he learnt that it was ordered he should be in London on Saturday evening, he expressed much concern thereat, as it would oblige him to travel on the Sabbath-day of the religion he had embraced. Lord George, we understand, first became a Jew while he was in Holland, and ever since he has resided in this place has been a very strict and rigid observer of every rite, ceremony, and custom of the Jews except that of attending the Synagogue, where he feared to appear lest he should be discovered. His beard he has suffered to grow to a considerable length, which, together with his dress, contributed so much to disguise him that he frequently went out in the daytime, though most of his hours were spent in his lodging-room in reading, writing, and learning the Hebrew language.”
His friend Robert Watson, MD, published his Life of Lord George Gordon: with a philosophical review of his political conduct in 1795. He discusses why Lord George converted to Judaism, at least as much as he far as he could guess:
Most interesting of all is the following exchanges between Lord George and a Jew named Angel Lyon. Gordon would only allow bearded Jews to visit him in prison. It seems that the latter was a poor Jew who had wanted to visit him for some assistance, but was refused entry because he did not have a beard. He sent him a letter:
“My Lord,—Although I was unfortunate, not to meet your Lordship’s approbation, on account of my beard being short; yet, I trust, your Lordship, when you consider the sending of Samuel to the house of Jesse, to anoint a king, and his being surprized that seven of the finest princes should pass before him unapproved, that God told Samuel, ‘ People see with their eyes, but I see the heart.’—Just so, my Lord, is my situation— a true Jew, the son of misfortune, having seen better days in many parts of the world; but am now hardly able to maintain myself, wife, and children, with all our endeavours united.—Your Lordship’s charitable disposition numbers of our tribe speak daily of, and I hope God Almighty will put it into your heart to add to the number of your good works by relieving the troubled mind of, My Lord, your Lordship’s most obedient and most humble servant,
now humbly waiting your Lordship’s answer,
AT No. 17, NORTHUMBERLAND ALLEY,
FENCHURCH-STREET, June 15,1789.”
-- and this is Gordon's reply; as you can see, after explaining his point of view, placing the blame for British Jews being shaven on rich Jews, etc., he advised him to grow his beard, and then he'd be glad to receive him (printed in the Public Advertiser October 16, 1789):
Apparently this was quite persuasive, as Angel Lyon wrote the following letter back (and see how he signs it now!):
“Gar-Zadak, My Lord, The letter which your Lordship thought proper to honour me with, will for ever merit my best acknowledgments, as I am thereby made superlatively happy, in finding that the solid reasonings therein perfectly agree with my heart, and are conformable to the directions of Almighty God, and to the sacred writings of our prophets.
“I beg leave to declare for myself, that owing to your Lordship’s few words and personal example, on the tenth of June, when I had the honour of waiting upon you, I took the resolution not to shave any more, which the bearer can personally testify.
“The whole of your Lordship’s letter is so just and strictly true, it will not admit of any reply, especially from one who is overwhelmed with trouble. Yes, my Lord, I will tarry readily, in the hope of seeing your Lordship, unless it shall please God to relieve me from it before the time
“I beg leave to subscribe myself with the greatest respect, your Lordship’s obliged,
Ashur Bar Judah.“June 24, 1789."
Blake's Jerusalem (1804) included many beautiful engravings. This is the frontspiece, and it is assumed to have been inspired by the dress of Lord George Gordon
Finally, here are three clippings from the Jewish Chronicle. The first is from 1867:
The second concerns a Jewish friend of George Gordon. Lord George made his acquaintance because of the curious --and noble-- inscription on his home, which read כל דכפין ייתי ויכול, dated 7.03.1896:
The third concerns a point I once made about time. The correspondent, writing in 1905, remembers that his grandfather told him all about the Gordon Riots of 1780.