Doug,
Gilmerstown is probably not a townland as such. But a subset. And the name may well have disappeared over the years. If you have a close look at the Griffiths maps of townlands in Dromore where Gilmours lived you might see the place mentioned (in small print).
Gilmerstown sounds a classic subset of a townland. Ulster is dotted with places like that. I know of an Adamstown in Roughan townland in Co. Antrim. A group of Adams families settled there and eventually it became known as Adamstown even though that was not the townland name. Gilmerstown is likely the same. Unless someone reading this knows where it is, pouring over a contemporaneous map is possibly the only way to find it.
Oughill doesn’t sound like a subset. It sounds like an alternative name for a townland. I have had a good look at the tithes and other records for the mid 1800s but can’t see what it might be. There is a townland called Oghill in Ballyclog but that’s right on the other side of Tyrone. It doesn’t seem likely.