I have both the AGI/IGI. Your certificate will say GROUND INSTRUCTOR:
ADVANCED INSTRUMENT
you have already taken the IGI portion; the AGI is the same except you turn in your results from the AGI written exam. The test bank of questions for that exam come from the Flight Instructor knowledge exam.
Just take the results to the Flight Standards District Office (FSDO) and they will issue you a certificate, as long as you already hold a current ground instructor certificate, Certified Flight Instructor certificate, or results from the Fundamentals of Instructing (FOI) written exam.
Good Luck,
Jason