Let $|L_g|$, be the genus $g$ du Val linear system on a Halphen surface $Y$ of index $k$. We prove that the Clifford index $cliff(C)$ is constant on smooth curves $C\in |L_g|$. Let $\gamma(C)$ be the gonality of $C$. When $cliff(C)<\lfloor{\frac{g-1}{2}}\rfloor$ (the relevant case), we show that $\gamma(C)=cliff(C)+2=k$, and that the gonality is realized by the Weierstrass linear series $|-{kK_Y}_{|C}|$, which is totally ramified at one point. The proof of the first statement follows closely the path indicated by Green and Lazarsfeld for a similar statement regarding K3 surfaces.