Magento2.1.x

I had a problem adding a product Image Label programatically. Here's the code I used:

 $imageLabel1 = 'Test1';
 $imageLabel2 = 'Test2';
 $imageLabel3 = 'Test3';
 $product->setMediaGallery (array('images'=>array (), 'values'=>array ())) 
 ->addImageToMediaGallery('/test1.jpg', array('image'), false, false);
 ->addImageToMediaGallery('/test2.jpg', array('thumbnail'), false, false);
 ->addImageToMediaGallery('/test3.jpg', array('small_image'), false, false);

Where i can set $imageLabel1 - 2 - 3?

Try this:

$imageLabel1 = 'Test1';
$existingMediaGalleryEntries = $product->getMediaGalleryEntries();
foreach ($existingMediaGalleryEntries as $key => $entry) 
 $entry->setLabel($imageLabel1);

$product->setStoreId(0);
$product->setMediaGalleryEntries($existingMediaGalleryEntries);

UPDATED

The code above is the only way to set label for a media gallery entry, because there's no direct API from Product to set them.

It also means, you still using your current code to set new media gallery, then use the code above to update label for it after $product->save() (another mean, you have to save() product twice)

Pseudo code

public function addImageToMediaGallery(Product $product) 
 // your code
 $product->save();


public function updateImageLabel(Product $product, $label) 
 // my code
 $product->save();


public function updateProduct() 
 // ....

 $this->addImageToMediaGallery($product);
 foreach ($labels as $label) 
 $this->updateImageLabel($label);
 

 // ....