I don't use a variety of characters, so it's tough for me to make an informed comment, but I think Tidus is deserving of a top tier. His moveset is evasive, quick and quite ranged at times.

My main, Squall, is probably mid tier, maybe even low tier, because of his poor HP attacks. His Bravery attacks are awesome though, so much so that you could call it broken. So I don't know where he'd belong.

I think you're bang on right about Jecht. Good balanced character with extreme range and tracking on several of his attacks.

But Gabranth for me seems low tier to me. I know he can still be deadly in the right hands (as with many other games and their low tier characters) but if the same guy was using Gabranth and Jecht, he'd definitely have a much easier time winning with Jecht.