Spoilers
With Sidney I could sort of understand. She had her own family now and she and Dewey would have probably drifted apart long ago if they hadn't been connected by all the killings
But Gale should have been way more devastated. Gale and Dewey were given as much screen time as Sidney in Scream 2, and they even got their own theme song (though it was ripped from the Broken Arrow soundtrack). And in Scream 3 I would argue that Gale and Dewey were the primary protagonists
I get that having scenes of Gale and Sidney bawling over Dewey's casket would seriously slow the movie down, but they barely seemed fazed at all by his death. Didn't feel true to the characters at all