Hi there,
As per our support policy, adding an extra reply takes you to the back of the queue, which is why you haven’t received a reply yet.
The download custom field is to output a download button on blog/single post listings, and wouldn’t be possible within the gallery lightbox functionality. It may be possible with some customisation, and while we’d love to be able to support every customisation request, we simply don’t have the time. We recommend that you seek a freelance developer if you need that functionality, potentially from one of the below resources:
https://codeable.io
Home
http://www.microlancer.com
Hope that helps.
– Ed