There were two Gideon Allen in Fairfield, CT, in the 1700's. The first was Gideon Allen (Lt. Gideon Allen, of Fairfield), son of Gideon Allen of MA and Milford, CT (Gideon Allen of Milford), and grandson of George Allen of Sandwich, MA (George Allen, of Sandwich). This Gideon moved to Fairfield in about 1699-1700. The other Gideon Allen was married in 1699, lived in Guilford, CT, and moved to Fairfield in 1702.
A few years ago, I started a project on Gideon Allen of Guilford and Fairfield, trying to find his ancestry. Although I found a possible line back to England, there was a gap in the records so I could not prove the idea. It is possible that Gideon of Guilford and Fairfield came from the Long Island, NY, but the records in Long Island in the 1600's are limited.
As a part of the project, I prepared a genealogy for Gideon and some of his descendants. Given common confusion between the two Gideon Allen families of Fairfield, I decided to add a file (https://www.geni.com/documents/view?doc_id=6000000082684694361) in the sources of the profile of "Gideon Allen, b. Sandwich MA d. Milford CT" (Gideon Allen of Milford). Hopefully, this file will help other uses to distinguish between the two Gideon Allens.