Gambit is described as 'Cross-platform chess game'. There are four games similar to Gambit for a variety of platforms, including Linux, Windows, PortableApps.com, Self-Hosted solutions and WebKit. The best alternative is Lucas Chess, which is both free and Open Source. Other Gambit like games are XBoard (Free, Open Source), PyChess (Free, Open Source) and 3D Hartwig chess set (Free, Open Source).
XBoard, also known as WinBoard on Microsoft operating systems, is a free graphical user interface client developed by Tim Mann that is compatible with various chess engines that support the Xboard Communication Protocol such as GNU Chess.