Gramps has both Families and Roles for Events that build Consanguinity and Affinity relationships implicitly. Then Associations for explicitly creating other relationships.
GEDCOM 5.5.1 (pg26) also has an ASSOC tag. (Or in the Association Structure GEDCOM7)
GEDCOM spec
Other associations or relationships are represented by the ASSOciation tag. The person’s relation or association is the person being pointed to. The association or relationship is stated by the value on the subordinate RELA line. For example:
0 @I1@ INDI
1 NAME Fred/Jones/
1 ASSO @I2@
2 RELA Godfather