@Override
    protected MatParam adaptMatParam(MatParam param) {
        if (param.getName().equals("alpha")) {
            String alphaMode = (String) param.getValue();
            switch (alphaMode) {
                case "MASK":
                    // "MASK" -> BlendMode.Off
                    getMaterial().setFloat("AlphaDiscardThreshold", MASK_ALPHA_DISCARD);
                    break;
                case "BLEND":
                    getMaterial().getAdditionalRenderState().setBlendMode(RenderState.BlendMode.Alpha);
                    break;
            }

            // Alpha mode is handled here, so return null
            return null;
        } else if (param.getName().equals("doubleSided")) {
            boolean doubleSided = (boolean) param.getValue();
            if (doubleSided) {
                //Note that this is not completely right as normals on the back side will be in the wrong direction.
                getMaterial().getAdditionalRenderState().setFaceCullMode(RenderState.FaceCullMode.Off);
            }
            // FaceCulling is a RenderState not a Material Parameter, so return null
            return null;
        } else if (param.getName().equals("NormalMap")) {
            //Set the normal map type to OpenGl
            getMaterial().setFloat("NormalType", 1.0f);
        } else if (param.getName().equals("LightMap")) {
            //Gltf only supports AO maps (gray scales and only the r channel must be read)
            getMaterial().setBoolean("LightMapAsAOMap", true);
        } else if (param.getName().equals("alphaCutoff")) {
            getMaterial().setFloat("AlphaDiscardThreshold", (float)param.getValue());
        }

        return param;
    }
The default alpha discard threshold for "MASK" blend mode.
jme3-plugins/src/gltf/java/com/jme3/scene/plugins/gltf/PBRMaterialAdapter.java