theory11 — Magic Tricks & the World's Finest Playing Cards
Yes Gallium has been used in tricks before. The one I know of is a spoon that dissolves in hot water/liquid. Various applications for that.
NOTE - Gallium is NOT to be consumed. I can't remember the exact composition, but I'm pretty sure it's significantly lead and I know it is toxic. So if you do something like letting a spoon dissolve in coffee (a presentation I have seen) do NOT drink that coffee.
Also note - the melting point of gallium is quite low. Depending on where you live, you may only be able to use it during the coldest months of the year. I know a guy in CA who had to literally change the percentages of various components of gallium-based gimmicks because the atmospheric temperature was higher than the melting point of said gimmicks for most of the year.
Although it is not harmful in small amounts, gallium should not be purposefully consumed in large doses.