I've read the docs and forum and it appears there is no possible way to control tabindex's on the form. They are hardcoded by Gravity Forms and the browser simply does what the code says to do.
It would be nice to have the ability to be able to override the tabindex for forms. Without it, forms have to be constructed to fit Gravity Form's rules and not ours. And, yes, I understand this is a function of the browser but since you have hardcoded the tabindex's, you might consider a way for us to override them. OR, have Gravity Forms save a hardcoded page and allow us the ability to modify it and then save it as the form's template.
Just an opinion since I'm fighting tabindex's and design on a 100+ input form.