Accuweather explains the difference between a gustnado and a tornado in the video below. The big difference is the gustnado is not attached to the clouds above it like a tornado. It spins upward from the ground. A gustnado is usually short-lived.
NOAA defines a Gustnado as "A gustnado is a small, whirlwind which forms as an eddy in thunderstorm outflows. They do not connect with any cloud-base rotation and are not tornadoes. Since their origin is associated with cumuliform clouds, gustnadoes will be classified as Thunderstorm Wind events. Like dust devils, some stronger gustnadoes can cause damage."