If you are thinking about taking a gap year after grade 12, it is best to follow the normal application process as though you will be studying immediately after school. Do everything as a regular applicant, including writing placement tests, submitting a portfolio, etc.
Defer your application
This way, if you decide to take a gap year all you need to do is request that your application be deferred, but you are already in the system for the following year.
Please note that any offers already made to you in the current year – for study, scholarship or residence – cannot be deferred. Deferred applications compete for a place with other applicants in the year to which the application is deferred.