Here is the full list of all 22 Gun Man achievements.
get_a_coin
level_1
level_2
level_3
level_4
level_5
level_6
level_7
level_8
level_9
level_10
level_11
level_12
level_13
level_14
level_15
level_16
level_17
level_18
level_19
level_20
level_21