That is actually really difficult to answer, both because of the lack of proper scale in the games and the different sizes of Pokemon. I would imagine it varies depending on the Pokemon, but it we assume they're around the same sort of size increase for the sake of the argument...
I'll use G-Max Grimmsnarl as an example, because it's the first one I found when I Youtubed the animation for G-Max Pokemon. When the camera pans out, Marnie is about as tall as G-max Grimmsnarl's foot. I'll make the assumption that Marnie is around 150cm tall, or 4'9". Height for protagonists ranges from 4'5" (Leaf) to 6' (Calem) so it's a...moderately conservative estimate. It's not likely to be TOO far off, and 150cm is a nice number to work with. G-Max Grimmsnarl is roughly 6 Marnies high, which is 900cm/29'5" (rounded) and regular Grimmsnarl is actually only slightly taller than I assume Marnie to be - 149.86cm, or 4'11".
So, my grossly simplified, very sketchy estimate would be that Gigantamax Pokemon are approximately 6 times larger than their regular-sized counterparts. If I am even remotely close it is likely to be 5 times bigger, because it's a more common multiplier to use in pretty much everything, but that's my best guess. It's poor. But it's got *some* basis at least? xD
That said, the animation shows the Pokemon expanding twice, so it's very possible that they're just 3 times larger...which is another common Japanese multiplier, because Char Aznable.
