Whether you're programming with a GC or without one it all comes down to profiling and optimization (when required). Using a GC means you need to think about memory allocations and reduce them (when required) because they factor into how long the GC will need to run. C# is a great language for this because it provides a lot of options for removing GC memory allocations from your code entirely (struct types, Span<T>, etc).

Not all GCs are created equally either. Unity, for example, is based on an ancient version of Mono and so it uses the Boehm GC which is significantly slower than the one used by .NET. Godot probably has two GCs because it primarily runs GDScript (their custom language) and only supports using .NET in a separate engine build. They'll all have their own performance characteristics that the developer will need to adjust for.