I've read that liquid retin a (tretinoin) used to be used on geograpic tongue, is this a safe treatment?

No treatment needed: Geographic tongue does not require treatment. However if it's causing discomfort, such as a burning sensation, i'd go see your dentist and typically a prescription of mary's magic potion/mouthwash should help!

Not recommended: I believe Retin-A should be kept away from the eyes, the mouth, angles of the nose, and mucous membranes. All medications have some potential risk and since geographic tongue is a benign condition that does generally not require treatment, i would not recommend using liquid Retin-A for that purpose. Ask your own dentist or physician for advice. Make sure what you have is truly geophraphic tongue.