Have you seen his original marriage record by any chance? It might well give his father's name which would help locate his birth/baptism in the IOM (which you'll know, seeing you've visited, isn't in England). The IGI have 4 William Gales around that period. The ones from 1816 & 1818 are extracted from parish records, the 1808 suggestion looks like it has been submitted by another researcher who has worked back from known information in the same way as you have, and is not a record.

It's also possible the surname might be spelt differently in the parish records - Gayle/Gail/Gaile/Gell all being possibilities (all being pronounced the same!). It would also hep to know where abouts in the island he came from, although known siblings would help too.