To men and women who served in the World War II and the people they liberated【状语】 ,//the G.I. 【主语】//was【系动词】// the common man【表语】//【who has省略】 grown into hero【后置定语】.

参考译文：对于那些曾在第二次世界大战中服役的男兵们和女兵们以及被他们所解放的人们而言，G. I.就是由普通人成长起来的英雄。(they指代men and women)

解答:grown into hero为过去分词作后置定语修饰前面名词“the common man”相当于定语从句“who has grown into hero”

总结：网友的理解是正确的。