You sure they didn't get confused and think you meant an actual leopard lol! I wouldn't feed beef to a gecko. They probs could digest it but why risk it?
Quite honestly, I don't think they'd even eat it. Leos are attracted to feed, at least to some degree, because of the movement of the prey item. I haven't ever seen a beef cube move except during a food fight.
When I was younger I did have one eat a piece of turkey, but it was the same gecko that would try to take chunks out of everything even if it didn't move. I have a nice scar from him.