The FunctionImport 'GetEmployee' could not be found in the container 'CtxDb'

Qualify the function import with the container name like this:

public virtual ObjectResult<Employee> GetEmployee()
        {
            return ((IObjectContextAdapter)this).ObjectContext.ExecuteFunction<Employee>("EntityContainerName.GetEmployee");
        }

The entity container name is found on your EDMX - right click anywhere and do "Properties".

source

Alternative way:

public virtual ObjectResult<Employee> GetEmployee() {
     return this.Database.SqlQuery<Employee>("GetEmployee"); 
}