Here is the full list of all 23 Gnomes Garden achievements.
For speeding up workers
For getting the first seed
For completing 5 levels with 3 stars
For restoring a plant
For constructing a building
For clearing one roadblock
For growing a lily pad
For getting the second seed
For getting the third seed
For getting the fourth seed
For getting the fifth seed
For getting the sixth seed
For getting the seventh seed
For getting rid of a Merrymaker
For stopping time
For restoring Stonehenge
For restoring the signal fires
For activating an altar
For speeding up construction
For saving the king
For completing 15 levels with 3 stars
For completing all levels with 3 stars
For collecting all trophies