/.../
model = assetManager.get("mesh/Sting-Sword-lowpoly.obj", Model::class.java)
texture = assetManager.get("badlogic.jpg", Texture::class.java)

val m = ModelInstance(model)

val mat = Material()
mat.set(TextureAttribute.createDiffuse(texture))
m.materials[0] = mat

/.../

attribute vec3 a_position;
attribute vec3 a_normal;
attribute vec2 a_texCoord0;

uniform mat4 u_projViewWorldTrans;
uniform sampler2D u_diffuseTexture;

varying vec3 v_normal;
varying vec2 v_texCoord;

void main()
{
    v_texCoord = a_texCoord0;
    v_normal = normalize(a_normal);
    gl_Position = u_projViewWorldTrans * vec4(a_position, 1.0);
}

uniform sampler2D u_diffuseTexture;
varying vec2 v_texCoord;

void main()
{
    //Get color
    vec4 color = texture(u_diffuseTexture, v_texCoord);

    gl_FragColor = color;
}