Uses for gold depend on your level in the game. At the beginning, you’ll need it to train troops, explore more areas on the map and, last but not least, to attack the Blackguard.
Later on, you’ll use Gold to upgrade your troops, your mines, and gunboat abilities in the Armory.
Additionally, once you’ve unlocked your Submarine (HQ level 9), you can use gold to send it on missions to recover sunken treasures!