Here is the full list of all 75 GIBZ achievements.
Unlock all hats and facewear
Die a lot (unlocks the Tall Top Hat)
Kill 600 enemies with fire (unlocks the Black Gas Mask)
Dodge roll 2,500 times (unlocks Dark Aviator Goggles)
[Halloween] Find and unlock the Spitter and Werewolf Masks
[Halloween] Smash 100 Pumpkins (unlocks the Pumpkin Head)
[Halloween] Eat 200 pieces of candy (unlocks the Witch Hat and Nose)
[Xmas] Find and collect the Antlers off the "Deerwolf" (unlocks Antlers)
[Halloween] Go Trick-or-Treating wearing a Halloween themed item
[Xmas] Get a Snowman Head
Hit a stick of dynamite 7 times back and forth with a friend
Heal 5 hearts using the heal dance on normal difficulty or higher (unlocks the Custom Cap)
Kill a Crusher by only hurting it with it's own rocks until it gets angry, then proceed to not take any damage from it (unlocks the Dark Crusher Mask)
[Base Defence] Clear 200 Waves
Kill 1,000 enemies with Supa Snipers
Use the Dynamite Launcher to launch yourself backwards rapidly (unlocks Combat Helmet)
Unlock every weapon
Headshot 300 zombies with sidearms (unlocks the Custom Bandit Mask)
[Base Defence] Clear 100 Waves
Chop down some stumps (unlocks Wood Axe)
[Base Defence] Have a Boss reach 0 health without a single player damaging them
[Base Defence] Don't let a single chicken damage anything during a wave of chickens
Kill 1,000 enemies with explosives (unlocks Mining Helmet)
[Base Defence] Clear 20 Waves
[Base Defence] Have 4 fully decked out Survivors in your base
Use the Lever-action rifle to teach some big monsters a lesson (unlocks the Black Cowboy Hat)
Cut off 1,000 limbs with the Bone Chainsaw
Beat the final boss having never lowered the difficulty below HARD or spent any money on that character (unlocks Dark Knight Helmet)
Beat the final boss having never lowered the difficulty below HARD on that character (unlocks Broadsword)
Kill a total of 25,000+ enemies
Cut up a bunch of fish (10) in a row without being hit by one (unlocks Headband and Blindfold)
Earn a total of £50,000
Hit back 262 projectiles (unlocks Baseball Bat)
Give the Machete Slinger a taste of his own medicine (unlocks the Bloody Metal Mask)
Clear the Tower Bridge, having all the bosses chasing you at once
Kill 2,500 enemies with melee weapons
[Base Defence] Unlock everything under the Survivor Tree
[Base Defence] Unlock everything under the Building Tree
[Base Defence] Ring the bell several times in a row
Defeat your friends (unlocks Hood)
Amputate 8,000 zombie arms
Find a great photo opportunity
Decapitate 10,000 zombies
Found and unlocked the Taller Top Hat
Revive 5 bosses (unlocks the Army Hat)
Beat the final boss having never lowered the difficulty below EASY on that character
Beat the final boss having never lowered the difficulty below NORMAL on that character
Play 10 mini-games (unlocks the Pith Helmet)
Kill 2,500 enemies with shotguns
Win a game of Defence without taking any damage
Kill 1,000 enemies with Supa Shotguns
[Xmas] Get a Festive hat off a Zombie or a Friend (unlocks Santa Hat)
Shoot off 5,000 pairs of zombie legs
Open 300 crates
Kill 2,500 enemies with single-shot rifles
Defeated the Wizard
Headshot 400 zombies with single-shot rifles (unlocks the Dark Steampunk Goggles)
Killed the Skeleton Goliath
Dig 750 holes
Play Zombie Surfing 10 times
Max out all stat buffs
Kill 2,500 enemies with auto-rifles
Have 30+ maggots on you at once
Kill 1,000 enemies with Supa MGs
Kill 500+ zombies in a single round of Mayhem (singleplayer)
Unlock all perks
Clear the Tower Bridge
Killed the Surgeon boss
Killed the Goliath boss
Kill 600 enemies with sidearms
Killed the Glutton boss
Killed the Fisherman boss
Repel a lot of zombies (unlocks the Plague Doctor Mask)
Kill 10 zombies with one shockwave in Zombie Surfing
Killed the Machete Slinger boss