Market Gnome's last ability will go onto the stack above the craft ability that caused it to trigger. This means you'll gain 1 life and draw a card before the transformed card returns to the battlefield.

Most of the time, Market Gnome's last ability will trigger because it was exiled as one of the components used in the craft ability. However, in some uncommon cases, the ability will also trigger if Market Gnome is exiled while you're activating mana abilities as part of paying for that craft ability.