Yeah, I was half-asleep when I posted that. You can’t since you can’t boot with your new card.
Since you can’t otherwise boot into the LiveISO on your machine, you will need to boot it up with the 6800 card you replaced. Then install and update Garuda and replace the card with the newer one.
Or ask @dr460nf1r3 to post an updated ISO. 
(Or just install Arch.) 