To nurture and sustain a brand effectively, you must be constantly in touch with what your stakeholders (consumers, users)) think about your brand and what you (your organization) thinks of your brand. Brand image is defined as the set of actual associations the consumer has with a brand; Brand identity is defined as the set of aspirational associations the organization would like to have of its brand.
The image-identity gap analysis capture the brand associations from these two perspectives (internal + external). It gives you insights to what extent the ideal identity match with the actual identity, physical identity and image.