Maybe it's because he is more used than other characters. I mean, if people doesn't bring astarion he can't die, right? so he doesn't push the "astarion deathcount". Same goes for shadowheart, Wyll or Zae'sel or whatever the gith is called. If people aren't using some characters is just logical they're not gonna die. And the more they use it is also logical there's gonna be more of that character dying, just because it's a probability
I have found Gale to be the nicest and most interesting companion, so i'm glad to have him in the team. However, he doesn't seem to be dying that much for me, Astarion has died way way more than Gale, at least on my playthrough
