Traceability Matrix:

Coverage Completeness can be achieved by creating a 
Traceability Matrix.

Traceability is the means of tracking down each individual 
requirement in the SRS to various work products created 
during the course of the Project.It describes the link 
between different stages of the Project Item – from its 
requirement to a configurable stage. 
Creating a Matrix is to prove that the System requirements 
are the requirements desired by the user.

Traceability Matrix is of two types:

Backward Traceability:
Backward Traceability to the documentation and other work-
products created prior to the requirements phase.

Forward Traceability:
Forward Traceability will depend upon how each requirement 
in the SRS is labeled / numbered. Forward Traceability is 
extremely important, as this is one of the ways of tracing 
a requirement to its implementation.

In short, the requirements are traced as below:
a)Requirements traced to Database tables.
b)Requirements traced to Program Specifications
c)Program Specifications traced to Test Cases
d)Test Cases traced to Program Code

So As a whole You can say by traceability Matrix you can do 
the Gap Analysis.