The Shach in YD Siman 117 Sif-Katan 13 writes,
דאין במתנה משום גניבת דעת
With regard to a gift there is no problem of "genevas daas" (deception).
The Shach writes this in regard to selling a not-Kosher piece of meat to a gentile and saying that it is in fact Kosher (since we see from SA Choshen Mishpat Siman 228 Sif 6 that there is a problem of genevas daas in business dealings with a gentile).
What is the reason and source behind what the Shach writes: why is a gift different from selling, that there isn't a problem of genevas daas? As well, is what the Shach writes true also of a Jewish recipient of the gift? (Seemingly it would be if the rule of genevas daas is the same for all people, Jew and Gentile alike.)