This is how the item "Everstone" works:
I gave Gastly the EVERSTONE.
I leveled Gastly up to level 25.
Gastly didn't evolve.
I took the EVERSTONE from Gastly.
I leveled Gastly up to level 26.
Gastly evolved into Haunter.
So basically the everstone just doesn't let the Pokemon evolve when it is supposed to. When you remove it and level up the pokemon that was supposed to evolve earlier it will evolve when a new level is grown.