We present GC-as-a-Service (GaS), a cross-runtime, cross-language garbage collection (GC) library that can be used to simplify the implementation of runtime systems, and that exploits available multicore technologies. GaS decouples GC from other runtime components and exposes a fine-grain API for use by GC-cooperative runtimes of different programming languages for heap memory management. GaS provides concurrent, on-the-fly GC and avoids moving objects for use as a precise or conservative collector. We integrate GaS within production-quality runtime systems for Python and Java. Our experimental evaluation shows that using GaS as an alternative to tightly integrated GC introduces modest overhead and that GaS reduces pause times significantly for Python and Java programs.