Gangrene can have different causes. It doesn't have to be started by an open wound. It can be caused by a systemic bacterial infection, or impaired blood flow to the affected area (i.e. in diabetic patients' feet for example). My guess is Griff had a systemic infection. My Millie had one, and she lost most of her toes as a result. I got really lucky and I was able to arrest the spread of the infection with Baytril and a topical antibiotic, but it was touch and go for a while.
I understand your feelings that it's time to give up. I've been there, and I did give up for several years. But I couldn't stay away from collareds for forever; they're too firmly nestled in my heart. My suggestion is that you keep on with what you have left, and if you lose them all, give it some time before getting any more. I don't know what bugs killed all my lizards, but after years of letting my cages sit empty, and then a thorough cleaning with bleach and scalding hot water, I managed to get rid of the pathogens, and all my new collareds have been fine.
You'll get through this, I promise.
élan

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5.6.0 Collareds
1.2.0 Leopard Lizards
1.0.0 Mali Uromastix
1.2.0 Green Anole
1.0.0 Chinese Dwarf Newt
1.0.0 White's Treefrog
1.0.0 Fire bellied toad
1.0.0 Treefrog (Alabama native)
Even More Felines
1.3.0 Canines
1.0.0 Equine