Galangal certainly is not grachai (or krachai/kra chai) in Thai. Kha (Ka or Kaa) is the Thai word for Galangal - as in Tom Kha Gai soup (literally "boiled galangal chicken" soup.
Grachai/Kra Chai is key ginger, sometimes known as lesser ginger or chinese key. It is a vital ingredient in the Thai dish of Khannom jeen nahm yaa but is not a viable alternative to galangal.
Also ginger is not really a good substitute for galangal as they have different flavours, you can't make Tom Kha with ginger for example. I'd recommend using dried galangal, not powdered, over ginger if fresh galangal is not available. Galangal is peppery'er' than ginger with less of a raw heat in its uncooked form.