The width of tires you should use for gravel biking depends on the type of gravel roads you will be riding on and your personal riding style.
For gravel roads that are relatively smooth and well-maintained, a tire width of 28-35mm is a good choice. These tires will provide a good balance of traction, comfort, and rolling resistance.
For gravel roads that are rougher and more technical, a tire width of 35-45mm is a good choice. These tires will provide more traction and stability on rough terrain, but may have more rolling resistance.
For extremely rough gravel roads or off-road adventures, tires width of 45mm or wider may be a good choice. These tires will provide maximum traction and stability but have more rolling resistance.
It's worth noting that wider tires provide more comfort and traction but can slow you down on smooth gravel roads. Narrower tires will roll faster on smooth gravel roads but may not provide as much comfort and traction on rough terrain. It's also important to consider the width of your rims and the size of your frame when choosing tire width.
Ultimately, the best tire width for you will depend on your riding style and the type of gravel roads you will be riding on. It's a good idea to experiment with different tire widths to see what works best for you. Some riders prefer to run tubeless tires to increase puncture resistance and reduce rolling resistance.