Mandatory

• Standard bike (no motor/electronics). We recommend Salsa bikes and Velocity USA wheels.
• Bicycle helmet
• Cue sheets
• Mobile phone
• Front handlebar light or helmet/head lamp (mandatory for any section when you will be riding at dusk or later). Or both.
• Rear flashing light

Highly Recommended

• Backup front light (ideal if other light fails) or extra batteries
• Wide tires. 700x45c or more is what we suggest due to sandy sections. 2″ or more for mountain bikes. For mid- to back-of-the-packers, keep in mind that some roads will be chewed up and made looser by the hundreds of bikers in front of you. When in doubt, go wider.
• Spare tube(s). For longer events carry at least two tubes – throw one in for tubeless if you have a gash.
• Pump / inflation device / CO2 cartridges
• Tubeless tire sealant
• Multi-tool or allen keys such as a 4mm, 5mm and 6mm. A multi-tool with a chainbreaker and spare chain link are useful items to have
• Tire levers
• Something to make a tire boot (seal a tire gash). Park Tool offer a boot product; alternatively, wrappers from energy bars or even money
• Extra derailleur hanger?
• Duct/gorilla tape. Wrap around bike part or popsicle stick
• Some zip ties for emergency repairs
• Dependable hydration system. We recommend Orange Mud products.

Source: http://www.gravelcyclist.com/1-useful-pointers