Because you acquired the home by a gift, your basis usually is the basis that your parents had in the home (what they paid for it, plus capital improvements they made over the years). It may be difficult to reconstruct this information, but their initial cost may be available (e.g., published on Zillow).
A revenue ruling is the Commissioner’s “official interpretation of the interpretation of the law” and generally is binding on revenue agents and other IRS officials. Taxpayers generally may rely on published revenue rulings in determining the tax treatment of their own transactions that arise out of similar facts and circumstances.