How did Saint Gabriel the Archangel become a saint?

"Saint" is synonymous with the Word "holy," and is sometimes used that way. Angels are given the designation "saint" without any canonization process, and have been called "saint" ("Sanctus" in Latin; "hagiOS" in Greek) since before there WAS a canonization process. Holy individuals and martyrs were venerated and called "saint" before the Church instituted the concept of formal canonization.