Is 'Kimi ga yo' a valid japanese predicate?


I'm currently learning Japanese and struggling at understanding the
translation of the japanese anthem 'Kimi ga yo' supposedly translated as 'Your
reign'. I heard about double subject construct so I initially thought it would
be the case but as the sentence is followed by 'wa', I couldn't understand how
it would be a complete valid sentence by its own since yo is not a verb but a
noun and the subject particle 'ga' need a verb to apply to. Why the sentence
is not 'Kimi no yo' ? (possessive particle).

Thanks

