Gehant (to my knowledge) has no connection to GUYON or GUYANT. The Gehant's came from France. There were three brothers that came here settled in West Brooklyn, Illinois around 1854.
The 'G' is hard- like 'Giant'. Some have changed the spelling to 'Jehant' to avoid mispronounciations.
Have you searched out or verified his birthplace in the UK?
You can also search for his name on ship records. Good luck in your search.