There's no guarantee that the later Harpers from Gamlingay (no B) were connected to the earlier ones. If you continue to follow your line back in Hatley St George it may lead to Gamlingay.
You need the PR for Gamlingay to establish which Harpers baptised in Gamlingay were buried there - it's noticeable that of the eleven Harper baptisms 1630-70 only three were male. If they all died young there's your reason! But one of them, John, appears to have married, perhaps twice, but he baptised no children in Gamlingay. Did he die after 1670 or did he move away from Gamlingay?
Another possibility is that there were Harpers in Gamlingay in the 1700s but they were non conformist so weren't baptised in the established church. But as there are no Harper marriages either I don't think this is too likely.
Don't forget that Gamlingay is only a mile along the road from Bedfordshire, and a mile from Huntingdonshire to the north, so there could have been movement cross county as well
David