Gun-Nac is viewed from above in a continuously scrolling vertical playfield. The objective is to survive each stage, destroy enemy formations, collect money and power-ups, and defeat a large end-of-stage boss.

The ship fires continuously while the player dodges bullets and terrain hazards. Several weapon types can be upgraded, and bombs provide a powerful screen-clearing attack. A shop between stages lets the player spend collected money on equipment, creating a light resource-management layer alongside arcade shooting. The campaign is divided into multiple stages with distinct enemy patterns and bosses.

Prioritize survival over greed when enemy fire becomes dense. Collect money whenever the route is safe, then use the shop to strengthen the weapon you rely on most rather than spreading upgrades too thinly.

Save bombs for crowded passages, surprise attacks, or bosses. Learn each boss's movement pattern and keep the ship moving in small, controlled arcs instead of making large, predictable sweeps.