You usually need garbage collection because you are allocating in the first place. And allocating and releasing adds some non-determinism. You apparently don't

You usually need garbage collection because you are allocating in the first place. And allocating and releasing adds some non-determinism. You apparently don't know how much exactly needs to be allocated - otherwise you wouldn't have opted for the allocations and GC. That non-determinism translates to a non-determinism in CPU load, as well as to "I'm not sure whether my program can fulfill the necessary timing constrains anymore". So I kind of would agree that producing any garbage during runtime makes it much harder to prove that a program can fulfill hard realtime guarantees.