Duels are against npcs, not real people, so no one will report you.
But to be safe I've never used game guardian before year 3. You can safely get your attributes above what you need for the story by using the gem/coin hack from this modded apk to buy a few items from the cosmetic shop. It's only at about year 3 where, if you are using the energy/gem hack to progress through the story quickly, that you will outpace the required stats so using game guardian is a way arround that, rather than farming lessons hundreds of times.
As I said earlier, I've used game guardian for over 2 months now. First on a throwaway account, and then on my main (Which is now on year 7 chapter 56) once I was more confident that I would not get banned.