Yep you should be able to get into the Centra Ruins to get GF Tonberry still
To get Tonberry you've got to fight around 20 tonberrys there and you can't leave the ruins or you will have to start over again.
When you've thought at least 20 Tonberrys the Tonberry King should appear, defeat him and you will get GF Tonberry.
I found that Squall's Lion Heart limit break came in handy when doing this, also I think when I went against the Tonberry king Squall only did his Lion Heart limit break and I defeated him
