I can't figure out why you haven't returned the light already, upon realizing that leopard geckos do not need UVB exposure. Those bulbs aren't cheap...I would go get your money back and put it towards something more useful.

If you are looking to add light to the cage during the day for viewing, using a normal, non-UVB florescent tube will be fine.

If you don't mind me asking, what pet store told you this?