Svirfneblin already get 20% resistance to fire, cold, lightning, acid, and energy, and 33% resistance to poison and disease.

Svirfneblin
sc damage
Bash______: Vulnerable____________-33%__Pierce____: Normal
Slash_____: Normal______________________Fire______: Moderately resistant___20%
Cold______: Moderately resistant_____20%__Lightning__: Moderately resistant____20%
Acid______: Moderately resistant_____20%__Poison____: Resistant_____________33%
Negative__: Normal______________________Holy______: Normal
Energy____: Moderately resistant_____20%__Mental____: Normal
Disease___: Resistant______________33%__Drowning__: Normal
Light_____: Normal______________________Radiation_: Normal


Gestalt metamorphosis carries over the 20% to fire, cold, lightning, and acid (and drops the 20% energy), PLUS the 33% poison and disease to their form when they shift.

Diamondback gets -33% cold, and +33% fire, PLUS 90% poison.

(Diamondback BEFORE gestalt metamorphosis)
sc damage
Bash______: Normal______________________Pierce____: Normal
Slash_____: Normal______________________Fire______: Resistant______________33%
Cold______: Vulnerable____________-33%___Lightning__: Normal
Acid______: Normal______________________Poison____: Nearly immune__________90%
Negative__: Normal______________________Holy______: Normal
Energy____: Normal______________________Mental____: Normal
Disease___: Normal______________________Drowning__: Normal
Light_____: Normal______________________Radiation__: Normal

The carry over from gestalt metamorphosis is suppose to stack with whatever resistances the form has, as demonstrated by the diminished vuln cold while in diamondback form, dropping from -33% to -13% after the svirf gives a 20% boost.

(Diamondback AFTER gestalt metamorphosis)
sc damage
Bash______: Normal_____________________Pierce____: Normal
Slash_____: Normal______________________Fire______: Resistant______________33%
Cold______: Slightly vulnerable______-13%___Lightning_: Moderately resistant______20%
Acid______: Moderately resistant_____20%___Poison____: Nearly immune_________90%
Negative__: Normal______________________Holy______: Normal
Energy____: Normal______________________Mental____: Normal
Disease___: Resistant______________33%__Drowning__: Normal
Light_____: Normal______________________Radiation__: Normal

However, notice that the fire resist and poison resist aren't stacking properly anymore. It's been a while since I played a svirf diamondback, but I'm quite certain I remember fire resistance going up to 50% in diamdonback form after the 20% boost from svirfneblin gestalt (with diminished returns putting it at 50% rather than 53%). I forget exactly what the diminished returns does to the diamondback's 90% after the svirf 33%, but for some reason the number 95% is sticking in my head.

I've also had a svirf shifter with vulture form, which provides 33% disease resistance, and after gestalt, I recall it going up as well. I just can't remember the exact numbers on that one. It's been too long.

And the 20% Energy resistance does not get carried with gestalt metamorphosis by design, so that's fine.