Gigabit Wi-Fi refers to the upcoming 802.11ac Wireless LAN specification that will support data transfer speeds of 433Mbps per spatial stream and up to 1.3Gbps speeds for three-antenna (three stream) hardware.
Also sometimes called "5G Wi-Fi," Gigabit Wi-Fi operates only in the 5 GHz frequency range (where there is currently far less potential for interference), as opposed to the 802.11n spec, which operates in both 5 GHz and 2.4 GHz ranges. Gigabit Wi-Fi products will offer backwards compatibility with previous 802.11 specifications, including 802.11n.
The 802.11ac specification is expected to be officially approved in 2013 by the IEEE, and draft 802.11ac "Gigabit Wi-Fi" products are already available for purchase. Additionally, newer smartphones like the iPhone 5 and Samsung Galaxy III currently support 5GHz Wi-Fi and are Gigabit Wi-Fi-capable.