So is the Georgia Giant nothing else but a CNBG x GSF? -- no

It is a form (some type)of hybrid lepomis. The exact makeup is unknown to all except the developing hatchery.

I would call the place they came from and ask exactly what they sold you. After they answer I would ask if they sell Georgia Giants or know about them. It is possible that they have stumbled on a near Georgia Giant mix on their own.

If you want a put and take HBG pond that is a good use of HBG like fish. If you want a balanced LMB/BG type pond then add adult CNBG and fish out some of the existing hybrids.