The word "Gillion" is spelled with two "L's" due to its phonetic transcription in IPA, which is /ˈɡɪljən/. The doubled "L" represents the sound of a "L" in the middle of the word that is pronounced with a short vowel sound, as in "pill" or "hill". The additional "L" helps to distinguish this sound from the pronunciation of a single "L", which typically represents a long vowel sound, as in "feel" or "heel". Therefore, the spelling of "Gillion" accurately reflects its pronunciation.