Question: what is the difference between g# and ab?
Answer: In general, it is the same note. Choosing Ab or G# indicates the scale. Ab can indicate the Eb, Ab, Db, Gb and Cb scales while G# can indicate A, E, B, F# and C# scales. In classical composition it also implies the direction of the specific voice, G# means going up (probably to A) while Ab means going down.