Here is the full list of all 8 Gon' E-Choo! achievements.
Finish stage 255
Finish stage 255 without cheating
Eat the gator in 9 levels consecutively
Finish 3 stages in a row with a hug
Save yourself by collecting the last paper airplane in a level
Finish 3 levels in a row without losing a life
Enter only symbols as your initials.
Enter some very special initials