Grandma Smurfette is actually Hogatha in the Season 6 episode "The Gallant Smurf". Hogatha turns into Grandma Smurfette in order to capture Grandpa Smurf and show him to the queen so she will make the prince marry her.
She is not to be confused with Nanny Smurf because they have a similar name, and not to be confused with Snorty Smurf because they are both Hogatha as a smurf.