The rare-earth tritelluride GdTe3, in both bulk and flake forms, exhibits intriguing properties, including high carrier mobility, superconductivity, and charge density wave transitions. In this study, the axial and dimensional effects of the van der Waals antiferromagnet GdTe3 in the ab-plane on its cryogenic magnetic properties were systematically explored. Temperature–magnetic field phase diagrams for GdTe3 under magnetic fields aligned along the a- and b-axes (H||a and H||b) were constructed, revealing significant ab-plane anisotropy and the emergence of a novel magnetic state. Magnetization curves for H||a and H||b further confirmed this anisotropy. The magnetic susceptibility χ(T) of thin and thick samples showed distinctly different ground states under H||a. In addition, the separation of zero-field-cooled and field-cooled curves observed at 7.0 K suggests a spin density wave linked to an incipient antiferromagnetic order oriented along the c-axis. These findings demonstrate that the magnetic ground states of GdTe3 can be tuned by controlling the material’s dimensionality at cryogenic temperatures.