Much of this route is great for a gravel bike. However, I highly recommend you take a MTB for this one, not a gravel bike, due to chunky sections on 152A, 9513 and 9551.
152A and 9513 is also known as the Diamondback Gulch UHV trail, and is classified as an advanced UHV / 4x4 trail; it is much better suited for a MTB. The north section (152A) can be very slick and may be impassible when it rains. Includes steep, rocky pipeline on the west end (9513). Suitable for most high-clearance 4x4s.
Similarly, 9551 is classified as an intermediate UHV / 4x4 trail. Known as the Outlaw UHV trail, this loop has several steep, rocky sections, and bedrock shelves to climb. Scenic views from top of mesa of red rocks and Black Mountain. See ancient ruins and pictographs at Honanki Heritage Site or Palatki. Suitable for most high-clearance 4x4s.
If you do take a gravel bike, be prepared to do some walking on the rougher sections.