Recently my dad and I got into a debate over a Spotify advert which said "Get 3 free months of Spotify Premium". My father said that it was incorrect and should be "3 months on Spotify Premium Free" as "free" should follow whatever is free. I felt it sounded alright and nothing was wrong. Like "3 free gifts" sounds more correct than "3 gifts free".
Which is correct? Is this something where different rules apply on spoken and written English?