Removal of steering gear box

That is not information that is printed anywhere. We just have to grab wrenches and try them until we find the right one. I can share that on a truck as new as yours, 15 mm is common, but GM is also the only manufacturer that used some 16 mm stuff. If it was not for GM, our 16 mm wrenches and sockets would never get used. You may find a 17 mm used on a few applications too, but I do not think I have ever run into anything larger than that. The nuts on return lines are usually a little smaller, but I would be surprised to find something smaller than a 15 mm on a truck.

Be sure to use a line wrench, (aka "flare nut wrench"). The nuts on power steering and brake lines are made of real soft metal. A standard open-end wrench will usually round off the corners of the nut. When you do not plan on reusing the old line, you can cut it off right next to the nut, then use a six-point socket and a ratchet.