Gravel is a block commonly found in the Overworld or in the Nether in large quantities. It's one of a few blocks affected by gravity.
It can be mined with any tool, but the shovel is the quickest. One can also use an Excavator to generate large amounts of gravel and flint quickly.
When mined, there's a 10% chance of dropping flint, which is used to make flint and steel, arrows and many Tinker's Construct tool parts. Fortune enchantment or Luck modifier added by lapis lazuli increases chance of dropping flint to 14%, 25% and 100% when mined with Fortune I, Fortune II or Fortune III, respectively. When mined with a tool that has Silk touch enchantment or the Silky modifier, gravel will always drop itself and will never drop flint.
Another use of gravel is to make Grout, which can be smelted in a furnace to produce a seared brick. Seared bricks can then be used to craft the main components of the Smeltery.
As a Tinker's Construct material:
Handle modifier: 0.7
Full tool durability: 119
Mining speed: 5.25
Mining level: 1 (Iron)
Base attack: 1 Heart
Flint can be used to make quite good early-game tools and weapons, being effectively a better version of stone (albeit one without the stonebound ability). Nevertheless, it is better to use wood, bone or cactus to make tool rods, due to the flint's handle modifier being less than one, and therefore weakening a tool.