From Gambit wiki
The Snow package system
Snow is a package management system for R4RS and R5RS Scheme. Nearly all Snow packages run on Gambit. Users can simply download and install existing packages. For those who want to develop new packages, Snow has its own documentation.
Black Hole module system
Scheme Spheres is a structured collection of R5RS-compatible modules and tools for building applications with Gambit, along with a growing documentation base. It is built on top of Gambit's namespace functionality. Supports syntax-rules and rsc-macro-transformer (define-macro-like system that plays well with syntax-rules). The list of features is extensive, as an example take a look at the Core sphere. Start using Scheme Spheres with the quickstart guide.
There is an informal listing of Gambit libraries and code samples on the Dumping Grounds.