The Relationship Calculator works fine when individuals are connected via known ancestors and descendents. However, with many DNA cousin matches, there is no genealogical path for the Relationship Calculator to work. DNA testing services, DNA Painter, and other software methods exist for estimating likely familial relationships.
Gramps should consider formally adding an new Association category for “Alternate Relationships”, where researchers can enter the various DNA estimates, along with the source and statistical percent certainty for each such estimate.
I know how to “ad hoc” add new Associations to the appropriate Gramps python file, but display and reports of such information is beyond my ability.