Create a button that resets the scene by putting the background color to black and makes the cube bounce back to its original scale. Add the button to the GUI:

Evas_Object *btn_reset;
btn_reset = elm_button_add(inner_box);
elm_object_text_set(btn_reset, "Reset");
evas_object_smart_callback_add(btn_reset, "clicked", btn_reset_cb, gld);
evas_object_size_hint_align_set(btn_reset, EVAS_HINT_FILL, 0);
elm_box_pack_end(inner_box, btn_reset);
evas_object_show(btn_reset);

Declare a callback that resets the variables that have influence on the drawing of the cube. In addition, animate the sliders when they get back to their original position using Ecore_Animator.

static Eina_Bool
animate_reset_cb(void *data, double pos)
{
    GLData *gld = data;
    double frame = pos;
    float x, y, z;
 
    frame = ecore_animator_pos_map(pos, ECORE_POS_MAP_BOUNCE, 1.8, 7);
    x = gld->slx_value * (1 - frame) + 0.75 * frame;
    y = gld->sly_value * (1 - frame) + 0.75 * frame;
    z = gld->slz_value * (1 - frame) + 0.75 * frame;
 
    elm_slider_value_set(gld->slx, x);
    elm_slider_value_set(gld->sly, y);
    elm_slider_value_set(gld->slz, z);
 
    return EINA_TRUE;
}
 
static void btn_reset_cb(void *data, Evas_Object *obj,  void *event_info)
{
    GLData *gld = data;
    gld->slx_value = elm_slider_value_get(gld->slx);
    gld->sly_value = elm_slider_value_get(gld->sly);
    gld->slz_value = elm_slider_value_get(gld->slz);
    ecore_animator_timeline_add(1, animate_reset_cb, gld);
}

The whole code: gl_2d.c