G.fast is a hybrid-fibre technology, which is designed to deliver Internet speeds of up to 1000Mbps over shorter runs of copper cable (up to around 250 meters via 106MHz+ of radio spectrum). The idea is that a fibre optic cable is taken closer to homes and then G.fast works to deliver the last few metres of service, which saves money because the operator doesn't have to big up your garden to lay new cables.
By comparison XG-FAST works in a similar way but via an even shorter run of copper and using frequencies of up to 500MHz. For example, XG-FAST delivered its top speed of 10,000Mbps by bonding two copper lines together over just 30 metres of cable. But this might be a problem for commercial operators, which will want to maximise profits by using more copper to reach more homes and not less.