Is it possible to make it so people need to post X amount of times before being able to see the gallery?
Not out of the box. You'd have to do some coding, and add a column to the users table to keep track of how many pictures each person has uploaded. You could also add a query to scan thru all the pictures to look for the id of the uploader and count each one, but it would be more efficient to add the column and update that field each time a person uploads a picture. Which would require more code.