Yes, GameGuardian is a completely safe application. It has support from a large community of users and developers on the Internet and none of them have warned that it contains malicious code. Therefore, your installation does not involve the risk of infection of our phone. However, its use may involve certain risks.
Although it can work without root access, it is recommended for this application to obtain superuser permissions. This is the easiest and most direct way to get the most out of its features. It could make your smartphone a less secure device, where applications have full access to the storage system. Also, getting root access in Android is a complicated process in many cases, and can completely disable your phone.
Also, some uses of GameGuardian are unethical. For example, it is often used to avoid micropayments or to help you beat other players illegally. This can cause you to be kicked out and lose your account, especially if you are an online video game user. Although GameGuardian pretends to be another game, the truth is that it is still detectable. Games like Clash of Clans, Need For Speed No Limits, or Clash Royale have systems capable of verifying whether a player is cheating or not.
In short, GameGuardian has a strong reputation in the community, and therefore, we can say that it is a safe application. However, keep in mind that although its installation will not bring you any risks, its use can have serious consequences, putting in danger your device's integrity or your profile within the online video game platforms.