Ok.
So I had way more trouble with this than I first thought.
And disclaimer....this doesn't work every time and I don't understand really why.
But here is what I did.
First you need to run GG in root (either proper root or root in the virtual space)
If not , I found I got heaps of junk results.
Again , I don't really understand.
Start tennis clash , then restart without protection.
Do a "auto" search for your coin value.
Play a match then search again for your coin value.
You should have 3 results left. So far so good.
Next , play a match and win. When the prize screen comes up , go back into GG and you'll see that your prize has already been added to your total. Here , modify the Q value to what ever you like.
Continue the game and you'll see your coin total has been changed. But wait, that's not everything. Now you'll need to make a purchase before you do anything else. This purchase will work , but it's likely after the purchase your total will reset to the original amount.
One purchase is better than nothing , right. ?
I'm going through it more and trying to get it working 100% , but it's a pain.