Support open ended dates (such “from” and “to” dates) will be new for Gramps 5.2 version. But the 5.2 GEDCOM import has not been revised to support them yet. So those dates will be imported as a text string.
There are some other date format that import as Text. So there’s a supertool script (described in another thread) for pushing “invalid dates” back through the Gramps date validation function. Running that script once 5.2 is released should fix those.