(I have a little experience with grills but none with squid.)
Gas grills have a temperature dial that usually goes up to 450-500F. Whether you actually achieve that temperature depends on a number of factors, but the top temperature is around there. A charcoal grill, on the other hand, doesn't have a temperature dial (temperature regulation is entirely up to you), but charcoal burns much hotter -- you can get 600-650F without too much trouble, and anecdotally I've heard of temperatures as high as 800F.
Squid apparently has some unusual cooking properties:
Not unlike putting laundry out to dry in a rain shower, grilling squid requires overcoming some very fundamental obstacles, namely that the surface of the squid will only brown once it has dried out enough, but the squid will overcook if it's exposed to the drying heat for too long. [...] The problem is that squid is a very, very wet creature with an incredibly high water content. (source)
More specifically, Peter Taylor added in a comment:
Squid and octopus have an unusual collagen structure which means that they toughen at low temperatures (60C / 140F). (Source: On Food and Cooking by Harold McGee).
By cooking at a higher temperature, you can cook off that high water content quickly, before the squid has soaked up so much heat that it's turned tough and rubbery.