Metal-assisted chemical etching (MaCE) of a (100) n-type GaP using patterned Pd catalysts in a mixed solution of HF and H₂O₂ at room temperature is reported for the first time. Various patterns of Pd catalysts, i.e., meshes and patches, with length scales ranging from 200 nm to several μm were used. Depending on the sizes of the Pd catalysts, GaP exhibits two distinctively different MaCE mechanisms: the conventional and inverse MaCE. With Pd nanomeshes, the ordered arrays of GaP nanocones were formed by the preferential removal of GaP directly under the Pd catalysts by the MaCE mechanism. When Pd micro-patches with several μm in length were used, bare GaP uncovered with the Pd patches was selectively dissolved to form GaP micro-mesa structures, following an inverse MaCE mechanism. We attribute these size-dependent etching behaviors to the dissolution limited etching characteristics of GaP during MaCE. Furthermore, we show that etched GaP structures can exhibit both mechanisms when a micro-patterned Pd nanomesh is used. The morphological evolution of etched GaP structures produced by MaCE is also presented.