When you attach a place record to an event without an event date, Gramps will display the place name based upon the hierarchy as if the date was 1 January, Year 1, the earliest possible date.
So in the record I show above, as the last entry, I have an enclosed by entry with no date information. This is how I set up a default path. If your last date entry in the enclosed by record is a
"before <date>", this is the path Gramps will select for a non-dated event. Something to keep in mind. The same is true for alternate names.
And Note: If Gramps has no valid option for the Name/Alternate for the date of the event, Gramps will insert a question mark (?).
Locations on a map do split into smaller entities or those smaller entities merge.
It just becomes a matter of creating a record for all these entities. It just becomes a case that the records we find will either never refer to that smaller entity that merged. The place name is never again found in an official record.
Dover, Maine and Foxcroft, Maine merged to form Dover-Foxcroft. After the date of the merger no official record will say just Dover, Maine.
The bigger issue are the churches and cemeteries that were in Dover now have to have Dover-Foxcroft added to their hierarchies. And the same is true when a place splits. Those churches and cemeteries need to reflect that they may have once been in the larger Andover, Massachusetts but now in the half of that town that became an independent town of North Andover.