This work presents a detailed study of GaSb quantum dot (QD) epitaxy on (001) GaP substrates by means of molecular beam epitaxy. Despite the large mismatch between GaP and GaSb, we show that in the nucleation-diffusion regime, the QD size distribution follows the predictions of the scaling theory. Scanning transmission electron microscopy analysis of grown QDs reveal that they are plastically relaxed by 60° pairs of misfit dislocations and the valence band offset measured by x-ray photoelectron spectroscopy on such QDs amounts to 0.5 eV. After capping, the QD morphology is strongly modified with a large P/Sb exchange-segregation reaction, which even leads to the formation of core-shell nanostructures. Remarkably the resulting QD layer is coherent to the substrate without any remaining misfit dislocation and exhibits still strong composition modulations.