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