2.2.3 [W3C-XSD] Section 3.2.12, gMonthDay
The specification states:
Since the lexical representation allows an optional time zone indicator, gMonthDay values are partially ordered because it may not be possible to unequivocally determine the order of two values one of which has a time zone and the other does not. If gMonthDay values are considered as periods of time, in an arbitrary leap year, the order relation on gMonthDay values is the order relation on their starting instants. This is discussed in Order relation on dateTime (§184.108.40.206). See also Adding durations to dateTimes (§E). Pairs of gMonthDay values with or without time zone indicators are totally ordered.
Indeterminate comparisons are considered to be determinate.