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