Here is the full list of all 75 GIBZ achievements.
[Base Defence] Clear 200 Waves
Use the Dynamite Launcher to launch yourself backwards rapidly (unlocks Combat Helmet)
[Base Defence] Have 4 fully decked out Survivors in your base
[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
[Base Defence] Clear 20 Waves
[Base Defence] Ring the bell several times in a row
[Xmas] Get a Snowman Head
[Halloween] Find and unlock the Spitter and Werewolf Masks
[Halloween] Go Trick-or-Treating wearing a Halloween themed item
Kill 10 zombies with one shockwave in Zombie Surfing
Killed the Skeleton Goliath
Decapitate 10,000 zombies
Shoot off 5,000 pairs of zombie legs
Max out all stat buffs
Kill a total of 25,000+ enemies
Unlock all hats and facewear
Kill 1,000 enemies with explosives (unlocks Mining Helmet)
Die a lot (unlocks the Tall Top Hat)
Dodge roll 2,500 times (unlocks Dark Aviator Goggles)
Kill 1,000 enemies with Supa Shotguns
Headshot 300 zombies with sidearms (unlocks the Custom Bandit Mask)
Cut off 1,000 limbs with the Bone Chainsaw
Amputate 8,000 zombie arms
Kill 2,500 enemies with melee weapons
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)
Earn a total of £50,000
Kill 600 enemies with sidearms
Kill 1,000 enemies with Supa Snipers
Kill 2,500 enemies with auto-rifles
Kill 1,000 enemies with Supa MGs
Defeat your friends (unlocks Hood)
Defeated the Wizard
[Xmas] Get a Festive hat off a Zombie or a Friend (unlocks Santa Hat)
Kill 2,500 enemies with single-shot rifles
[Xmas] Find and collect the Antlers off the "Deerwolf" (unlocks Antlers)
Kill 600 enemies with fire (unlocks the Black Gas Mask)
Unlock every weapon
Chop down some stumps (unlocks Wood Axe)
Headshot 400 zombies with single-shot rifles (unlocks the Dark Steampunk Goggles)
Hit a stick of dynamite 7 times back and forth with a friend
Repel a lot of zombies (unlocks the Plague Doctor Mask)
Found and unlocked the Taller Top Hat
Dig 750 holes
Find a great photo opportunity
Heal 5 hearts using the heal dance on normal difficulty or higher (unlocks the Custom Cap)
Use the Lever-action rifle to teach some big monsters a lesson (unlocks the Black Cowboy Hat)
Hit back 262 projectiles (unlocks Baseball Bat)
Revive 5 bosses (unlocks the Army Hat)
Cut up a bunch of fish (10) in a row without being hit by one (unlocks Headband and Blindfold)
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)
Unlock all perks
Win a game of Defence without taking any damage
Kill 2,500 enemies with shotguns
Clear the Tower Bridge
Clear the Tower Bridge, having all the bosses chasing you at once
Kill 500+ zombies in a single round of Mayhem (singleplayer)
Play Zombie Surfing 10 times
Play 10 mini-games (unlocks the Pith Helmet)
Open 300 crates
[Halloween] Eat 200 pieces of candy (unlocks the Witch Hat and Nose)
Have 30+ maggots on you at once
Killed the Surgeon boss
[Halloween] Smash 100 Pumpkins (unlocks the Pumpkin Head)
Killed the Goliath boss
Killed the Fisherman boss
Killed the Glutton boss
Give the Machete Slinger a taste of his own medicine (unlocks the Bloody Metal Mask)
Killed the Machete Slinger boss