Among the films starred by Nick Wahlberg, what is the percentage of the films with G rating?
'Nick Wahlberg' is a full name of an actor; full name refers to first_name, last_name; G rating refers to rating = 'G'; calculation = DIVIDE(SUM(rating = 'G'), SUM(rating)) * 100
SELECT CAST(SUM(IIF(T3.rating = 'G', 1, 0)) AS REAL) / COUNT(T3.film_id) FROM actor AS T1 INNER JOIN film_actor AS T2 ON T1.actor_id = T2.actor_id INNER JOIN film AS T3 ON T2.film_id = T3.film_id WHERE T1.first_name = 'Elvis' AND T1.last_name = 'Marx'