Susan,
I too have come to the conclusion that your James Watt Gilmour came from Scotland. I have checked the indexs for 1861 and can find no James or his father Thomas that could fit.
At that time Ramsey was an active port and James could simply have been working on the boats. If he were Manx would they have gone to Scotland so soon after marriage and then the child be bought up there?
Now you have Thomas's occupation as a manufacturer I wonder what he made. Could he have named his son after the engineer James Watt?
I am also a bit doubtful about the janitor could this be painter with an oddly written p?
I am pleased you got the certificate it did give you more information to go on.
Barbara