G-machine n. an automobile capable of withstanding one g of extra gravitational force due to braking and turning at high speeds; (hence) any fast vehicle, such as an automobile or boat. Etymological Note: G- < ‘gravity’ + machine (source: Double-Tongued Dictionary)
Further reading
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there is also virtual machine, called G-machine, to run lazy functional programs (like Haskell programs) on. See e.g. http://citeseer.ist.psu.edu/8740.html