Step 1: Recall the family gunmetal belongs to.
Gunmetal is historically a casting bronze, and bronzes by definition are copper alloyed primarily with tin, as opposed to brasses which are copper alloyed with zinc.
Step 2: Look at a typical recipe.
A classic gunmetal composition is roughly 88% copper, 10% tin, and 2% zinc, sometimes called red brass or LG2 in casting specifications. Even though a small amount of zinc is present, it is only added as a deoxidizer during casting and to slightly improve fluidity, a minor addition, not the defining alloying element.
Step 3: Why tin, not zinc, defines it.
The mechanical character of gunmetal, its pressure tightness, corrosion resistance, and good bearing properties used historically for cannons and valve bodies, comes from the tin dissolved in the copper matrix, exactly the same strengthening mechanism seen in true bronzes.
Step 4: Conclusion.
Since the principal and property defining alloying pair is copper and tin, gunmetal is correctly classed as a Cu-Sn alloy rather than a Cu-Zn, Al-Cu, or Al-Mn alloy.
\[ \boxed{Cu - Sn} \]