GDScript is not very maintainable as the code base grows. It lacks proper refactoring tools (e.g. the ones from Jetbrains Rider), static type checking, flexible object system and many 3rd party libraries which might be needed
You likely won't need to do manual memory management nor think about undefined behavior. If your writing basic c++ to handle the simulation in a game, it's going to be pretty orthodox and these problems likely won't manifest.
The purpose of recommending c++ here is:
If GDScript is too slow, reach directly for C++.
I'm specifically recommending GDScript over C# for ease of use and c++ over C# for performance.