Gravity: Low gravity makes it fly straight and high gravity makes it curve like grenades.
Bullet Speed: Tiles per frames default 1. Typically this only has to do with missions. Lower the number the faster the bullet travels. So 1 should typically be faster than 1.2. Though the difference will probably not seem like much.
Bullet Power: Bullet Damage
Bullets Frequency: Rate of Fire, delay between shots in seconds. Lower the number the faster the bullet shoots out of your weapon. So 0.17 is going to be faster than 0.2.
Bullets Distance: How far the bullet will go. Distance is number of tiles. A tile is 16x16 pixels. Typically this only has to do with missions.
Bullets Spread: The bullets are not shooting exactly where you point, it adds some randomness. It's in radiant (not degrees). We usually keep this low around 0.1.
Recoil: Obviously pushes you back when you shoot. Higher the number the more it pushes you.
Shooter Hitpoints: Shooter takes damage, don't think this actually works and is not being used currently.
Attack Radius: This is only for melees and it's the distance in which you can hit a player.
Frenzy Bonus: After you kill somebody with a weapon that has Frenzy it lets your weapon do +1-2 damage.