What is GT3 themes kits? It's a library of ready made sections from different pages. It allows you to insert any section available in this library to any page. Here is a quick guide how to do that.
First, please open the page where you want to insert the section in Elementor and scroll down and click this green icon.
A popup window will appear with the GT3 themes kits library. Select any of available sections from the library and click "Insert" button. The section will be added to the page content. That's all.