If "Mystical Space Typhoon" is chained to "Geartown", "Mystical Space Typhoon" resolves first and destroys "Geartown" at Chain Link 2. Then, "Geartown" resolves without effect as Chain Link 1. Since it still resolves, the destruction of "Geartown" is not the last thing to happen, so you miss the timing and cannot activate the effect of "Geartown