Nico - I've never seen a GT that takes 800m of line, even on 45lb line. I have had many GTs on light tackle over the years on lighter braid and while there is little doubt a big fish can spool you, we're taking 250-300m, not 800m.
If you're serious about targeting fish of the size you're describing, that's hard work. A fish of that size off the rocks is always going to be tough as hell. You can buy specific shore based GT rods which are specially made for this purpose. Companies such as Fisherman, Ripple Fisher and Medusa Custom Works have special rods that are 10 to 11 feet long that can handle PE8 braid. That said, these rods will be EXPENSIVE.
I'd personally be fishing no more than PE8 as you will not be able to properly utilise the power of the line while standing on the rocks. Maybe even PE6 is enough for this purpose and you would cast better with it. As Andrew asked, what distance do you need to get to the fish?