Hi Mat,
I am not familiar with EasyEDA and I don't have an account with them to test anything. However Google tells me the issue might be because the posted Gerbers from 2016 were from an older version of Eagle, and those might not import into EasyEDA. I have posted a new version of the Eagle Gerbers generated from Eagle v9 and you can try those and see if you get a different result. You can download them from the Downloads page that JHamm linked above. The file you are looking for is "Eagle Board and Schematic (zip)" under the heading of TCB Mk1 Hardware.
That being said, I don't know why you'd need to open them in EasyEDA. As far as I can tell, that is simply a board development tool, but you don't need to change the board. If you want to order PCBs you can use the links provided on the same Downloads page to order them from OSH Park or SEEED Studios. I believe the OSH Park Gerbers will also work at JLPCB and possibly other fab houses as well.
Finally - if your motivation to build a board is to save money, I can assure you from experience that you will be saving no money at all as opposed to just buying the board from Torro, even at their current price. The assembly process is also by no means trivial, you will need the means and ability to reflow, and then further equipment in order to get the bootloader onto the empty processor, etc... You will spend a very great amount of time at this task and even I who have done it many times was never certain to get a functional board after all that effort, there are many things that can (and will) go wrong along the way.
Torro has had their boards assembled in Germany, which is reflected in the cost, but then again, things made in a German factory are guaranteed to be of the highest quality! I would seriously recomend just buying from them.
Here is the link to Torro:
https://www.torro-shop.de/OPEN-PANZER-Tank-Control-Board_1
Luke