another's take on it:
“Shunkan’Idou”, the technique erroneously dubbed instant transmission directly translates to teleportation. Despite commonly being so in fiction, teleportation does not need to be instantaneous. Teleportation just means moving from one location to another without physically traversing the distance. Time can be elapsed during teleportation. In quantum mechanics, quantum tunneling, essentially real world teleportation, has a finite speed. Significantly faster than the speed of light, but limited nonetheless. While coming from non canon sources, Instant transmission is also shown to have an elapsed time between teleporting.
Even if it did directly translate to Instant transmission, the naming fallacy would also still apply. There would need to be a direct statement saying that the technique is instant from the manga or databooks. As far as I know, no such statement exists. That said, if Shunkan’Idou is instantaneous, it would not be an outlier. Dragon Ball characters have had an infinite level of reaction and combat speed since SSG Goku and Beerus sent shockwaves across not only an infinitely sized universe, but also the afterlife and kaioshin realms possibly existing in a separate space time continuum, in a finite timeframe.
Another note, Granolah’s, and by extension Gas’ teleportation ability being faster than Goku’s doesn’t necessarily have to refer to the transportation speed, but the start up time. Goku is rarely seen teleporting without his finger to his forehead during combat because it is incredibly difficult and takes time to focus before teleporting. It seems as though there is no or minimal lag between when Granolah and Gas choose to teleport and when they actually do, unlike Goku, which allows them to use it in rapid succession usefully mid fight and even against each other.