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