If you have not already found this, there was a daughter Marretje (Mary) born to Nicholaus Gerrebrantse and Marytje Cockefer on 13 Nov 1774, and baptized at the Aquackanonk Reformed Church on 11 Dec 1774. Witnesses were Pieter Gerrebrantse and Marretje Gerrebrantse. I see a total of six baptisms for this couple, all girls: 1773: Feebie (Phoebe) 1774: Marretje 1777: Jannetje 1783: Elizabeth 1786: Annatje (presumbably died young) 1788: Annatje
Note that there was another Nicholas Gerrebrantse who had at least four children with a Marritje Mourusse (Morris). This couple appears to have been slightly older--their children were born 1766-1780. While both spouses are the same, and the Cockefairs and Morrises were close (adjoining properties), and there were some similarities in the baptismal sponsors, I don't think these two Nicholases were the same man, because Mary Morris had a child 8 July 1772, and Mary Cockefair had one 31 Jan 1773. I have a hunch that the Nicholas Garrabrant who married Mary Morris was the son of Pieter Garrabrant and Elizabeth Van Wagenen, since those were the witnesses/sponsors of his first son, Pieter, as well as his daughter, Elizabeth.
I can't immediately identify your Nicholas Garrabrant, although I suspect he was the Nicholas Garrabrant born to Abraham and Marytje Vreeland on 6 Jan 1748 and baptized 7 Feb that year at the Second River Reformed church. Your Mary Cockefair pretty much had to be the daughter of either Alexander Cockefair b 1721 or John Cockefair b 1735, the only sons of the Alexander Cockefair who first settled in that areah. Of those two, I would guess the elder (Alexander b 1721), since he married Phoebe Morris, for whom their first child appears to have been named. There are several internet sources who combine this Abraham Garrabrant with (my) Abraham G. Garrabrant, but after a lot of research, I know that they were distinct individuals.
Genealogy from those families (and that whole area at that time) is challenging, but I hope this helps.