I'm learning how to make custom tracks for a game called BallisticNG and it requires me to use version 2.79 of Blender specifically due to a neccesary addon not being updated for 2.8. 

[![enter image description here](https://i.stack.imgur.com/Okc4q.png)](https://i.stack.imgur.com/Okc4q.png)

In the above image is the basic track I created within the game's track creator and exported as an OBJ for the purpose of adding terrain in Blender. My current goal is trying to fill the center hole along the top most edges of the inner wall. I've tried the Grid Fill option but for somereason it just doesn't want to work with this track.

File Here:
[![](https://blend-exchange.giantcowfilms.com/embedImage.png?bid=6781)](https://blend-exchange.giantcowfilms.com/b/6781/)