Here is the full list of all 21 Gnomes Garden: Christmas Story achievements.
For creating the second tree decoration
For creating the first tree decoration
For completing 5 levels with 3 stars
For constructing a building
For clearing one roadblock
Recruit 50 or more workers
Banish 100 or more gremlins in a game
For creating the third tree decoration
For creating the fourth tree decoration
For creating the fifth tree decoration
For creating the sixth tree decoration
For creating the seventh tree decoration
For speeding up workers
For stopping time
Recruit 50 or more merrymakers
Collect 50 or more sorceresses in a game
For speeding up construction
For completing 15 levels with 3 stars
For completing all levels with 3 stars
For collecting all trophies
For creating a Christmas tree