The phenomenon of gold shortage is present in Dragon Age: Inquisition and you can experience it especially during the first hours of the game. At the time, you will not have too much gold on you and each purchase that you want to make needs to be well thought over. One of good ways to obtain money is to sell all of the items that the party does not need anymore to the merchants. Specifically, all the weapons and armor elements (e.g. the one marked in blue or violet), as well as valuables whose only purpose is to bring you profit (the exception are the unique items dropped by the monsters which you can leave for research at the Inquisition HQ).
The reason for which you should visit merchants often is that your inventory is limited to 60 slots only and it can be filled really fast. However, make sure that you do not sell a rare weapon that one of the party members could benefit from using.
Another interesting way to earn money is to complete missions that you activate in the war room at the Inquisition HQ. However, this only goes for the quests that guarantee you gold (e.g. "Gather Coin), one of which has been shown in the screenshot above. The downside of this kind of quests is that you earn relatively little gold. Their upside, however, is that some of them can be repeated over and over again and that they are completed automatically. Therefore, once you receive an appropriate message about the completion of the mission by one of the Inquisition advisors, you simply need to return to the Inquisition HQ and claim your gold (and at the same time, task another errand).
Apart from the abovementioned, it is also a good thing to explore all the areas. Among others, you can find gold on the opponents and monsters that you kill, as well as in crates, containers and others.