The gathering skills are linked to specific crafting skills. To gather, find an approriate node for your skill in the world and right click it. You can also open the crew management interface to send crew members to gather items, although the yield is low, it does add up over time. So while you are questing, if your companion is not busy or not needed, send them on a gathering mission. Beware however that all missions do cost money, at lower levels around 100 credits, at higher levels it can go over 1000 credits per mission.