Here is the full list of all 75 GIBZ achievements.
Killed the Skeleton Goliath
Defeat your friends (unlocks Hood)
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 10 zombies with one shockwave in Zombie Surfing
Clear the Tower Bridge
Killed the Surgeon boss
Killed the Goliath boss
Found and unlocked the Taller Top Hat
Win a game of Defence without taking any damage
Killed the Glutton boss
Killed the Fisherman boss
Cut up a bunch of fish (10) in a row without being hit by one (unlocks Headband and Blindfold)
Killed the Machete Slinger boss
Earn a total of £50,000
Kill a total of 25,000+ enemies
Amputate 8,000 zombie arms
Shoot off 5,000 pairs of zombie legs
Decapitate 10,000 zombies
Kill 600 enemies with sidearms
Kill 2,500 enemies with shotguns
Kill 2,500 enemies with single-shot rifles
Kill 2,500 enemies with auto-rifles
Kill 2,500 enemies with melee weapons
Kill 1,000 enemies with Supa Shotguns
Kill 1,000 enemies with Supa Snipers
Kill 1,000 enemies with Supa MGs
Kill 500+ zombies in a single round of Mayhem (singleplayer)
Open 300 crates
Unlock every weapon
Unlock all perks
Max out all stat buffs
Play Zombie Surfing 10 times
Give the Machete Slinger a taste of his own medicine (unlocks the Bloody Metal Mask)
Kill 600 enemies with fire (unlocks the Black Gas Mask)
Repel a lot of zombies (unlocks the Plague Doctor Mask)
Headshot 400 zombies with single-shot rifles (unlocks the Dark Steampunk Goggles)
Use the Lever-action rifle to teach some big monsters a lesson (unlocks the Black Cowboy Hat)
Play 10 mini-games (unlocks the Pith Helmet)
Revive 5 bosses (unlocks the Army Hat)
Dodge roll 2,500 times (unlocks Dark Aviator Goggles)
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)
Die a lot (unlocks the Tall Top Hat)
Unlock all hats and facewear
Dig 750 holes
Have 30+ maggots on you at once
Find a great photo opportunity
Hit back 262 projectiles (unlocks Baseball Bat)
Kill 1,000 enemies with explosives (unlocks Mining Helmet)
Chop down some stumps (unlocks Wood Axe)
Hit a stick of dynamite 7 times back and forth with a friend
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)
Cut off 1,000 limbs with the Bone Chainsaw
Clear the Tower Bridge, having all the bosses chasing you at once
[Halloween] Smash 100 Pumpkins (unlocks the Pumpkin Head)
[Halloween] Eat 200 pieces of candy (unlocks the Witch Hat and Nose)
[Xmas] Get a Festive hat off a Zombie or a Friend (unlocks Santa Hat)
[Xmas] Find and collect the Antlers off the "Deerwolf" (unlocks Antlers)
Beat the final boss having never lowered the difficulty below HARD on that character (unlocks Broadsword)
Beat the final boss having never lowered the difficulty below HARD or spent any money on that character (unlocks Dark Knight Helmet)
[Halloween] Go Trick-or-Treating wearing a Halloween themed item
[Halloween] Find and unlock the Spitter and Werewolf Masks
[Xmas] Get a Snowman Head
[Base Defence] Clear 20 Waves
[Base Defence] Clear 100 Waves
[Base Defence] Clear 200 Waves
[Base Defence] Unlock everything under the Building Tree
[Base Defence] Unlock everything under the Survivor Tree
[Base Defence] Have 4 fully decked out Survivors in your base
[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
[Base Defence] Ring the bell several times in a row
Use the Dynamite Launcher to launch yourself backwards rapidly (unlocks Combat Helmet)