Where was this found? Are there artifacts there as well? Could be a polished rock out of the river, creek or shoreline, could be a glacially polished rock, could be an artifact but nothing about it says it is.
You would have to know something about the geology of the area where the rock was found. Just as likely to be a water polished rock or glacial polished pebble as a gastrolith. Gastroliths were collected by Native Americans; sometimes assumed to be part of a shaman's medicine kit.