Which prefix (Upsarg) will be used to make the word 'गमन' (Gaman) its antonym?

The word 'गमन' means 'going'. When the prefix 'आ' (Aa) is added to it, it becomes 'आगमन' (Aagaman), which means 'coming'. Thus, 'आगमन' is the exact antonym of 'गमन'.