From the homepage:
Gambit is a high-performance implementation of Scheme based on an optimizing compiler. It conforms to the IEEE-Scheme? standard (IEEE P1178) and the Revised^4 Report on Scheme (R4RS). It is almost R5RS compliant and is lacking syntax-rules. The full numeric tower is implemented, including: arbitrary precision integers (bignums), rationals, inexact reals (floating point numbers), and complex numbers. Several extensions to the standards are provided.
(this relates to version 3.0)
Gambit 3.0 was released in 1998, and Gambit 4 has been in development since then.
Gambit 4.0 was released on August 25th 2007. See http://www.iro.umontreal.ca/~gambit.
Gambit's website is now wiki-based, and Gambit's source is managed by Mercurial.