It is, yes, however it's a bit different to most other GC-based languages in that it's soft-realtime tracing GC. In addition, you can manage your memory completely manually if you like, and in-fact thats how you interface with C libraries, which I can say from experience works much nicer than I honestly expected going in to it.

Excellent reference regarding Nim's GC and what it does differently here: http://nim-lang.org/docs/gc.html