Magnetic topological materials hosting nonzero Berry curvature have emerged as a focus of intensive research because of their exceptional magnetoelectric coupling phenomena and potential applications in next-generation spintronic devices. In this work, we successfully synthesized high-quality GdAlSi single crystals, a prototypical member of $R\mathrm{Al}X$ ($R$ = rare earth elements; $X$ = Si/Ge) family that has been theoretically predicted to sustain a nontrivial Weyl semimetal state. Through systematic investigations of magnetic and transport properties, we identified two successive antiferromagnetic transitions at critical temperatures $T_{N1} \sim$ 31.9 K and $T_{N2} \sim$ 31.1 K, as evidenced by temperature-dependent resistivity, magnetic susceptibility, and specific heat measurements. Notably, applied magnetic fields exceeding 8 T induce a third magnetic transition ($T_{N3}$), generating a cascade of metamagnetic transitions that collectively form a dendritic phase diagram. This complex magnetic behavior is attributed to the interplay between localized $\mathrm{Gd}\text{−}4f$ moments and itinerant conduction electrons, possibly mediated by Dzyaloshinskii-Moriya interactions. Transport measurements revealed striking stepwise anomalies in magnetoresistance when crossing phase boundaries, accompanied by pronounced hysteresis loops arising from magnetic moment flopping processes. Our results not only establish GdAlSi as a rich platform for investigating correlated topological states, but also demonstrate its potential for engineering topological phase transitions through magnetic symmetry manipulation in Weyl semimetals.

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• Antiferromagnetism
• Electrical properties
• Magnetoresistance
• Topological materials