You mean garter snakes. They are mildly venomous
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There's no such thing as a "Gutter Snake", unless you're talking about the reticulated hose-like augers used to clean out gutter systems. You might be confusing it with a "Garter Snake", which are snakes of the Thamnophis genus. Or, the phrase might also be a corruption of the word "guttersnipe" which is an old slang word for a young vagrant.
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It must have fallen from the tree onto your roof top. Don't take chances.There would be more.