That final battle left him near death, and only extensive bionic treatment saved his life. To use an old cliché, he’s more machine now than man, but his mental state is also a far cry from the Gabriel of old.
After all that he’s been through, he’s more reserved, more pessimistic, and less likely than ever to bow to the authority of other Imperial factions like the Inquisition. As the Chapter Master he also feels the burden of responsibility more than before. The campaign on Cyprus sees him deployed with his old company, the 3rd, and he finds himself missing his days as their Captain. As the Master of all the Blood Ravens he now finds he can’t just wade in to fix every problem, or offer himself as a sacrifice for the wider victory. It’s a difficulty that becomes more and more apparent as the story progresses.