You can use Google Analytics to track conversions made through your galleries. If you already have Google Analytics set up for your Shopify store, you're already set up to track.
If you are not set up yet through Shopify, follow this Shopify Help Doc to get set up.

1. Log into your Google Analytics account

2. Click Acquisition on the left menu and then the Campaigns sub menu

3. Click All Campaigns and this will bring an overview of your campaigns

4. Click on the campaign titled products-gallery and that will show you specific numbers for covet-pics / popup