Here is the full list of all 21 Gnomes Garden: Halloween achievements.
Recruit 100 or more merrymakers
For collecting all trophies
Collect 50 of every resource on a level
For completing all levels with 3 stars
Collect 7 keys
Collect 100 of any resource on a level
For building seventh jack-o'-lantern
For building sixth jack-o'-lantern
For building fifth jack-o'-lantern
Recruit 100 or more workers
For building fourth jack-o'-lantern
For building the third jack-o'-lantern
For building the second jack-o'-lantern
For building the first jack-o'-lantern
For stopping time
For completing 15 levels with 3 stars
For speeding up construction
For speeding up workers
For completing 5 levels with 3 stars
For constructing a building
For clearing one roadblock