What the others said.
Few main reasons to get a GI
Don't need to have all the flight hours, basically anyone can do it.
Holds your FOI, if you start on your CFI but can't finish up, getting your GI will make it so you don't have to take the FOI test again.
You're getting your gold seal for your CFI, you have to hold a GI as well as meet the pass rates and volume, you end up with a gold seal and it counts as a reup for your CFI cert as well.
When I got my GI, I had already been a CFI, I just took the test for the flavor of GI I wanted, didn't have to take the FOI as I was already a CFI, brought in a breakdown of all the students I had recommended over the past couple years, filled out a 8710 for BOTH my initial GI and my gold seal add on / CFI renew, went to the FSDO where they verified everything, walked out with a GI, gold seal and fresh renew of my CFI, only thing I had to pay for was the GI written, quite a good deal as far as ratings and renewals are concerned!