Could be a throat, smoke chamber and flue behind that wall. Or maybe not. Anyway it looks pretty freestanding.
We recommend only using volcanic ash when installing a gas log set outdoors. Sand "packs" into clumps when exposed to moisture. Since gas takes the path of least resistance, you get no or low flame at the clumps and high flame in other places. It is easily fixed by loosening the sand to promote an even gas flow. However, volcanic ash is a better solution. Since the grains are larger and less uniform than the sand, they do not pack.
Everything else is the same.
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