Composite Key for Gizmo Tracking
as it will have three table 


one table for Owner,  Identity Auto-Increment by 1 will give OwnerId
one table for gizmos, Identity Auto-Increment by 1 will give gizmosId 
One table for mapping the many to many relationship as

OwnerId - FK
gizmosId - FK and 
 Other columns


Now as the combination of OwnerId and gizmosId will make a row unique you can create primary key constraints that will enforce Uniqueness as well.

ALTER TABLE schemaName.TableName
ADD CONSTRAINT PK_Relation PRIMARY KEY CLUSTERED(OwnerId , gizmosId);