If you take calcium and pour water on it you get hydrogen and calcium hydroxide. I should know. In an unsupervised high school chemistry experiment I used this reaction to create a hydrogen torch. It came to an unfortunate end when a stopper popped out of a flask and sprayed me with said calcium hydroxide.
Why not calcium since it doesn’t require gallium?
Pure potassium is much more fun when water is applied. Take one pound pure potassium, insert into a flask in pea sized chunks, insert a stopper with two holes, insert a glass tube into one to inject water onto the pellets, insert a glass tube in the other to carry away the hydrogen.
Use a flame hood when applying a bic lighter to the device.