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The Eu(Ga_{1−x}Al_{x})_{4} series is composed of centrosymmetric structures which exhibit a wide range of rich topological phenomena, including some members hosting magnetic skyrmions. In this letter, we investigate the previously unreported intermediate compound EuGa_{2.4}Al_{1.6}, which hosts two distinct phase transitions under zero applied magnetic field. We have used resonant elastic x-ray scattering with full linear polarization analysis to unambiguously determine the zero-field magnetic structures, which consist of a transition between a basal plane transverse spin density wave at higher temperatures into a noncollinear helical ground state. Furthermore, we demonstrate a phase coexistence regime below the transition and reveal an elliptically modulated helical magnetic structure emerging from wavevector splitting.