How to link gallery & products from different sites

I think I understand,

You have to link from the gallery on site B to the products on site A?

You can do that using the Custom URL fields for each attachment. On the gallery edit page, click the “i” when hovering over an image in the gallery. A popup will show, and on the bottom right there is a field for Custom URL, which you can set to point to site A.

Do that for your products. Then close the popup.

Then set the Attachment link to “Custom URL” in the gallery settings, and save the gallery.

Let me know if that worked for you

Hi Bradvin
Thanks for the answer.
When I hober over an image and I click the “i”, i don’t receive any popup!
I am redirected to the media page. (see image below)
On that page, there is a field “custom URL”

But, Where is the button “save” ? I never saw such a button for saving the galleries!!

Though I don’t saw any popup, You told me to link my gallery located on site B to site A.
But this is not working!
I want to link an element of site A to the gallery located on site B !
But how to give an URL to the gallery on site B?
Not from B to A, but from A to B!
That is the problem.

For being more clear:
On my site B, I have a gallery named “25”.
On my site A, I have in a product description a word “gallery”
I would like to place a link on the word “gallery” of site A to the gallery 25, located on site B.

Remark:
the gallery 25 cannot be relocated on site A.
I have more than 1000 galleries and the site B is a storage area for them.