Hey there,
I’m in the process of building a site using Flatsome and using Elementor for the shortcode widget.
My work around has been building the page in Flatsome UX Builder then copying the HTML from that page > create a new Elementor page, add a HTML section in a column and paste the HTML code from your Flatsome page into that.
If your Elementor Gallery is in the middle of the page, you’ll need to break the HTML code into two columns and put the gallery between the two HTML columns.
Hope this helps if you’re still working on it, or you if you’re reading this in the future.