Convert your native WP Galleries into Essential Grids

To convert your native WP Galleries into an Essential Grid,
add your regular WP Gallery shortcode inside an Essential Grid shortcode.

Example:
[ess_grid alias="test"][gallery ids="941,940,939"][/ess_grid]

Grid Setup:

When using a WP native gallery as an Essential Grid’s main source, the grid’s “category/source” can be anything, as it will always be overridden with the WP Gallery instead.  So just click the “Clear Categories” button as no official selection is needed.

Then further down the source tab’s settings, make sure the grid’s Media Source is set to “Featured Image”

Skin Setup:

Similar to traditional post-based grids, an Essential Grid with a WP Gallery as its source can include the image’s title and excerpt (aka “Caption”)

Result:

Additional Resources:

Creating a Native WordPress Gallery: 
https://codex.wordpress.org/The_WordPress_Gallery

Working with the Essential Grid Item Skin Editor:
https://www.themepunch.com/essgrid-doc/item-skin-editor/

Setting up the Lightbox for your Essential Grids:
https://www.themepunch.com/essgrid-doc/lightbox-options/