Given that their first child (Experience) was born Aug 16, 1761, have you considered the marriage of Robert Jr. Grey (Gray in the marriage intention) to Margeret (sic) Watt, Nov 18 1760 Brookfield? You can see a transcript here although I viewed the transcript on findmypast.com (Massachussetts Vital Records to 1850) and Ancestry no doubt has it as well.
FamilySearch has Massachusetts, Town Clerk, Vital and Towns Records, 1579-2001 images for browsing -- Brookfield is at https://familysearch.org/pal:/MM9.3.1/TH-1961-31537-32092-66?cc=2061550&wc=MMBX-FJW:n1448934938 -- and these may provide information about the parentage or residence of the bride and groom to confirm or disprove the hypothesis. However, the organisation of the information isn't chronological. There's an index book (entries separated by surname but not ordered by surname or date) for browsing at https://familysearch.org/pal:/MM9.3.1/TH-1961-31534-32645-56?cc=2061550&wc=MMBX-FJ3:n315312186, but you may be in for a long stint of browsing images.
Another angle of approach to investigate further would be to locate the marriage intention which might provide additional information or clues.
I'll add that you need to do an exhaustive search of the Massachusetts vital records for all marriages between a Robert Grey or Gray and with a spouse Margaret (I'd suggest from 1750 when Robert was about 16 to 1761 when their first child was born, and include all possible variants of both names) and assess each one to determine (1) is it geographically feasible; (2) what information is available in the marriage record or marriage intention to correlate with what is already known about "your" Robert and Margaret; (3) what records are available about each couple after they married, again to correlate with your couple of interest. You may end up with a single most probable marriage (or if you're lucky, incontrovertible evidence) or you may not. You will most likely need to apply the techniques of the FAN principle and Inferential genealogy to progress.