Here is the full list of all 22 Gnubbl achievements.
Successfully beat level 1
Successfully beat level 2
Successfully beat level 3
Successfully beat level 4
Successfully beat level 5
Successfully beat level 6
Successfully beat level 7
Successfully beat level 8
Successfully beat level 9
Successfully beat level 10
Successfully beat level 11
Successfully beat level 12
Successfully beat level 13
Successfully beat level 14
Successfully beat level 15
Successfully beat level 16
Successfully beat level 17
Successfully beat level 18
Successfully beat level 19
Successfully beat level 20
Successfully beat a level with the first try
Restart or die 25 times in one level