If you have a pre-existing condition, it will be excluded from the GAP cover for 12 months. Thereafter it will be covered. Please note that although this is the common practice, it is always subject to underwriting.
It is advisable therefore to submit your application and even if it is excluded you still may need the shortfall cover for something else! So get GAP cover soonest. Dont be like 50% of applicants who only get it after paying in a shortfall!