Sony’s apparently to have a big presence there, and some surprise (i.e., not revealed yet) game announcements are promised at Gamescom in general.
But that said, unless Sony’s announcing PlayStation 5 itself there, I would assume that if Testament (or whatever) is a PS5 launch title, it wouldn’t be announced until the platform itself was announced. Otherwise, it’s putting the cart before the horse.
I’m strictly a PC gamer these days and mostly playing smaller indie titles. Indy devs will be at Gamescom, but I’m sure all the videogaming news coverage will be hyperventilating over AAA game sequels (Gears of War et al).