Packages
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
Black Hole is a module system for incremental development and easily sharing code. It is bundled with a set of general purpose libraries, including several popular SRFIs.
Scheme Spheres
Spheres - A set of tools and libraries for real-life Scheme. Multiplatform and Mobile. Check out the code for updated instructions, or visit the project page.
Dumping grounds
There is an informal listing of Gambit libraries and code samples on the Dumping Grounds.