George Hipwell's baptism in 1743 can be found on the IGI, an extracted item, so reliable.
Hunts Marriage Index has a marriage of George Hipwell of Colmworth to Eliza Loveding at Gt Staughton, Hunts in 1768. It also has Thomas Hipwell marrying Susan Solding at St Neots, Hunts, at the beginning of 1740. Their daughter Jemima was the first Hipwell baptism in Colmworth in 1742 (the parish register starts in 1604). I see that George and Eliza baptised a son Solding Hipwell.
If you don't have Thomas Hipwell's baptism c1720 how do you know that he was the son of Gregory and Ann, as opposed to say, William Hipwell who was also baptising children in Eaton Socon at roughly the same time? Gregory Hipwell, carpenter of Honeydon, Eaton Socon, left a will which was proved in 1750 - Bedford and Luton Archives and Records Service has a copy under ref 1750/58. If you don't already have a copy it would be worth getting one to see if he left a bequest to a son Thomas or to grandchildren. The fact that Thomas and Susan baptised a son Gregory might be a pointer as to his father, but it's only circumstantial evidence - he might have been named for an uncle Gregory, especially as the eldest son was George, thus confounding the traditional naming pattern. It's inconsistent that Gregory should have baptised other children, but not Thomas.
Until you can prove who Thomas' father was it's premature to be researching Gregory's ancestors
David