Shape morphing with physics-based simulation naturally supports large deformations and topology changes, but existing methods suffer from a "rendering gap": nondifferentiable surface extraction prevents image losses from directly guiding physics optimization. We introduce PhysMorph-GS, which couples a differentiable material point method (MPM) with 3D Gaussian splatting through a deformation-aware upsampling bridge that maps sparse particle states (x, F) to dense Gaussians (mu, Sigma). Multi-modal rendering losses on silhouette and depth backpropagate along two paths, from covariances to deformation gradients via a stretch-based mapping and from Gaussian means to particle positions. Through the MPM adjoint, these gradients update deformation controls while mass is conserved at a compact set of anchor particles. A multi-pass interleaved optimization scheme repeatedly injects rendering gradients into successive physics steps, avoiding collapse to purely physics-driven solutions. On challenging morphing sequences, PhysMorph-GS improves boundary fidelity and temporal stability over a differentiable MPM baseline and better reconstructs thin structures such as ears and tails. Quantitatively, our depth-supervised variant reduces Chamfer distance by about 2.5 percent relative to the physics-only baseline. By providing a differentiable particle-to-Gaussian bridge, PhysMorph-GS closes a key gap in physics-aware rendering pipelines and enables inverse design directly from image-space supervision.

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