Hi there,
The Envira Gallery does not have such a feature to get the images and information from other sources other than the ones you set as a gallery and from Media section.
If you want to have a Grid or Gallery like feature which then can retrieve the featured image of a post then you would better consider the Essential Grid plugin:
The Essential Grid is a grid of images and information, but it also includes the Lightbox functionality. For more information:
https://www.themepunch.com/essgrid-doc/lightbox-options/
It is fairly complicated plugin and it takes time to learn the ins and outs but when you get a grasp of the details you will find it a helpful tool.
Thank you.