In British English, gnashers is a jokey word for teeth. Nobody uses it at the dentist or in a formal letter; it belongs to speech that wants a smile. You might hear “Look at those gnashers!” about a big grin, or “false gnashers” for a set of false teeth. The picture behind it is the verb gnash, to grind your teeth together, so gnashers are “the things you gnash with”. It is almost always plural. Younger people still understand it, though it can sound a little old-fashioned or cartoonish, like something a grandparent or a pirate in a comic would say.