Welcome to why I charge 2.00+ for my terrain textures
It's a lot of work and fine tuning to make them look good inside of Game-Guru. I have tools I use to make mine repeat cleanly but often it requires me going in and manually working on the file with paint.net and the clone stamp tool.
I'd highly recommend when you pick a tiled texture try to pick one without any special marks on it like cracks or holes because they just look awful.
I also make my own textures as well; simply go outside, point the camera down, click, crop to 1024x1024, run it through a filter to make it tileable, and then clean it up. Either way it's a lot of work.
I personally, if I was not in command of my own library of textures and had no idea how to do it I'd start with this guy's pack:
https://www.tgcstore.net/pack/10876
Mstockton is a decent guy and has really made this kit of his affordable to pretty much anyone. 1 dollar is a pittance.
Just buy that and save yourself the headache
Or if you want to buy mine, I recently reduced prices on them to be more reasonable. Though that said mine is 7.00 for the whole pack.
Best of luck either way.