Wordpress - How to get the first image gallery of a product in woocommerce in a loop

Along with the product thumbnail (I'm assuming you have this), what you need is a list (array) of the product images - WooCommerce has such methods, eg $product->get_gallery_attachment_ids().

You can grab the first ID in the array and use it to fetch the single image using wp_get_attachment_image(), or wp_get_attachment_url(), etc., then use that as an alternate source for the main (thumbnail) image.

Incidentally, the woocommerce_product_thumbnails call is outputting markup that you probably don't want to use. You'll need to either discard this or unhook functions from it to get the output you want.