hook_form_BASE_FORM_ID_alter() for node forms.

/**
 * Implements hook_form_BASE_FORM_ID_alter() for node forms.
 */
function custom_module_form_node_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $node = $form_state->getFormObject()->getEntity();
  if ($node->getType() != 'article') {
    return;
  }

  $form['field_image']['widget'][0]['#title_field'] = FALSE;
  array_unshift($form['actions']['submit']['#submit'], 'custom_module_form_submit');
   #multiple field field_image_gallery
  $i=0;
  foreach ($form['field_image']['widget'] as $item) {
    $item[$i]['#title_field'] = False;
    $i++;
  }
}
/**
 * Custom submit handler.
 */
function custom_module_form_submit(&$form, FormStateInterface $form_state) {
  $image_value = $form_state->getValue('field_image');
  $image_value[0]['title'] = $image_value[0]['alt'];
  $form_state->setValue('field_image', $image_value);
  #multiple field field_image_gallery
  $image_value = $form_state->getValue('field_image_gallery');
  foreach ($image_value as $item) {

  }
}