It looks like you're referring to an x64 version of the Gaztoof GMod-SDK , a software development kit often used for creating or updating cheats for Garry's Mod.
Gaztoof/GMod-SDK: A SDK with a built-in cheat for Garry's Mod. GMod-SDK-x64_[cheate....com]_
: Once injected, pressing INSERT typically opens the internal menu where you can toggle specific features. Important Considerations It looks like you're referring to an x64