The pango-gravity-hint enumeration defines how horizontal scripts should behave in a vertical context. That is, English excerpt in a vertical paragraph for example.

(define-g-enum "PangoGravityHint" pango-gravity-hint
  (:export t
   :type-initializer "pango_gravity_hint_get_type")
  (:natural 0)
  (:strong 1)
  (:line 2))  

:natural

Scripts will take their natural gravity based on the base gravity and the script. This is the default.

:strong

Always use the base gravity set, regardless of the script.

:line

For scripts not in their natural direction (e.g. Latin in East gravity), choose per-script gravity such that every script respects the line progression. This means, Latin and Arabic will take opposite gravities and both flow top-to-bottom for example.