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It is a shame that happened, but Geckos are not poisonous. The only thing I would be aware of is if it was a big Lizard it may cause a blockage. The cat may vomit or have diarrhea. If there is anything unusual you need to see your Vet. I would not anticipate any problems with your Kitty. Many cats eat lizards with no serious consequences.
I applied Frontline plus on my mail 10 year old cat