40 feet of rubber should be enough to launch a Gentle Lady to about 200', high enough to fly for a minute or two if there's no thermal activity. It won't launch a smaller lighter-weight glider very much higher unless the rubber was just not strong enough for the GL. You'd be just a well off to get a proper hi-start for your GL.
That said, there are some excellent built-up 60" gliders available. Look at the Chrysalis HLG by DJ Aerotech or the Lil Bird2 at skybench.com or the Zephyr 2 at isthmusmodels.com.
The Gambler at wrightbrothersrc.com is also very good.
If you think the problem might be a weak hi-start, get a small fisherman's scale and measure the pull of your rubberband when fully stretched. It should produce 5-8 pounds of pull to adequately launch your GL. If it doesn't, you can get a good quality hi-start from hosemonster.com or Hobby-Lobby. There's also the Dynaflite heavy duty hi-start at Tower Hobbies. Its cheaper in every way but will work for a GL or for the smaller models mentioned but don't stretch it as far as you would for the GL.
Good luck.