Sounds like a Northern Red-bellied Snake (Storeria occipitomaculata). They can be anywhere from tan to blackish gray above but they almost always have the bright reddish belly.

Search online for images and you should find one that looks like the one you saw.

And the other snakes are called "Garter" snakes, not Gardner. They are named (supposedly) for their bright colors reminding someone of colorful ladies' garters, not for their habit of showing up in gardens.
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Chris Harrison
San Antonio, Texas

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