Constraints which take comparable value types (e.g. greater_than) can be applied to strings and scalar type properties (see TOSCA simple yaml profile 1.3 ch 3.6.3.1.1):

3.6.3.1.1 Comparable value types
In the Value Type column above, an entry of “comparable” includes integer, float, timestamp, string, version, and scalar-unit types while an entry of “any” refers to any type allowed in the TOSCA simple profile in YAML.

The other types mentioned are already supported but SDC does not currently support for string and scalar types (see ConstraintUtil::checkComparableType).
This story is to add support for STRING