There may be other options at your disposal, however I will mention two options that you can explore to add a legend to your WMS on Geoserver.
You could always utilize the request
GetMap which will allow you to make modification/changes to your layout properties. You can read more on the Geoserver webapage here
Another option I came across is github user: kartoza webpage were he provides a simple WMS legend plugin to use.
This is a simple plugin for leaflet that will display a WMS GetLegendGraphic request result in a legend. At the moment the plugin is really, really simple in that is actually will display any image URI you pass it on initialisation. However that is enough to create a nice looking legend on your map like this: