I didn't say that GC was free ... malloc has overhead too.

For interactive programs, I would recommend two things to avoid (or hide) the pauses. Firstly schedule calls which run the GC when it won't be noticed: between frames in an arcade-style game, or just before accepting user input in a turn-based game. Secondly, size the minor heap large enough so that all computation between the scheduled GC runs can happen without invoking a minor sweep (but not too large that the minor heap is wasting memory -- some experimentation and tuning required).

I've written a couple of interactive games in OCaml that used this strategy and avoided visible GC pauses.