There isn't one single definitive Gaelic spelling for Kirsten. Gaelic (meaning either Scottish Gaelic or Irish Gaelic) doesn't have a direct equivalent name. The spelling would depend on how you choose to transliterate the sound of the name into Gaelic letters. You'd need to choose a similar-sounding Gaelic name or approximate the pronunciation phonetically.
To get a possible spelling, you'd need to specify which Gaelic (Scottish or Irish) you'd prefer, and then a Gaelic speaker could offer a plausible translation based on their understanding of the sound.