I personally wouldnt risk it, but alot of people take theirs out for pictures and such. Leos dont like bright lights or big changes I would think taking it outside would be stressful. I could be wrong as well though Ive never tried it.
Leopard geckos are not actually nocturnal, they are crepuscular as they are actually most active in low light, at dawn and dusk when it is not quite completely dark.
I'd say it would be alright on a cloudy or particularly dim day, just for a short period for pictures or something, but I doubt it would be an enjoyable experience as the wide open space can stress out your gecko