The Gunboat is one of your most important assets in battle. It provides fire support and battlefield supplies to your troops. Depending on its level it offers you several different weaponry items: Artillery, Flare, Medkit, Shock Bomb, Barrage, Smoke Screen and Critters.
Your Gunboat has a certain amount of firing power which you can find by tapping its little info button. Whenever you fire a weaponry your firing power decreases. How much firing power each item uses is marked within the little icon of the designated item.
Keep in mind that for each building you destroy you gain 3 firing power points. So you need to think strategically!
Last but not least you can upgrade your Gunboat weaponry in the Armory while upgrading your Gunboat will increase its firing power.