There are certain species of geckos/newts/anoles/skinks that can be toxic to cats and dogs. It is species specific and this of course depends on the area you are in. Depending on the type of critter ingested signs can be GI related with vomiting, neurologic, respiratory or systemic. It is best, of course, to try preventing your cat from coming into contact with these guys. The toxins are usually secreted through the skin so the lizard does not always need to be ingested to cause clinical signs. To know for sure what is in your area and how to identify it, I would give your local veterinarian or the extension service a call. They should know for sure how much of a risk your local "gecko" population is to your cat.
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