It's great to hear of another Le Gresley researcher, although I have just seen an indication that your Betsey Le Gresley - if she is indeed the one born 1797 in St John - might have been the daughter of the Jean Le Gresley who married Elizabeth Lucas. If so, she would not be a close relative of my line.
My Marguerite Le Gresley married my GGG-grandfather Jean Pepin on 7 Feb 1804 in St Helier. She had been baptized 23 Apr 1769 in St Peter, daughter of Jean Le Gresley and Elizabeth Perree. She had four siblings (that I know of): Elizabeth bpt. 18 Nov 1764, Susanne bpt. 9 Aug 1767, Jeanne bpt. 24 Sep 1771 and Amice bpt. 17 Nov 1773. All baptisms in St Peter.
Jean Le Gresley and Elizabeth Perree had married on 17 Jan 1760 in St Ouen. The marriage record describes him as "Jean fils Elie du Val de le Mare de St Pierre". I was told by a researcher at the Societe Jersiaise that this means Elie was from Val de Mare and son Jean was resident in St Peter at the time of his marriage; he was probably born in Val de la Mare, which is at the St Ouen-St Peter frontier, mostly in St Ouen. Jean was baptized in St Ouen on 13 Nov 1737; he also had an elder brother, Edouard, baptized in St Ouen 26 jan 1735.
Elizabeth Perree was the daughter of Josue Perree and Marguerite De Caen. She had been baptized 10 Dec 1735 in St Ouen and died 1787. This late death excludes the possibility that her husband Jean Le Gresley could have taken Elizabeth Lucas as second wife.
The identity of the Elie Le Gresley du Val de la Mare, the father of my Jean, is not quite certain. I was told by the same researcher at the Societe Jersiaise that it was most probably an Elie bpt. 23 Nov 1712 in St Ouen and buried 24 Jul 1763. Even more uncertain is his wife, although the site of the LDS mentions an Elie Le Gresley-Elizabeth Syvret marriage in St Helier on 24 jan 1735, which might fit the bill, timewise. (One is never sure if events in Jan and Feb have been corrected for the then valid assumption that the year starts on the first of March; in this case the uncertainty affects both the wedding and the birth of the first son, Edouard le Gresley).
I would be most interested to hear from you if you think there are points of contact between our lines.