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