When bases react with metals [[1]] gas is produced.

The surface is illusive, and the base has an unpleasant flavour. A salt is a base that has been decomposed in water. These mixtures result in salts when synthesised in response to acids. On red litmus paper, bases have been seen to become blue.
When the base interacts with metal, salt and hydrogen gas are produced. In response, metals like zinc release hydrogen gas in the form of sodium hydroxide.

The reaction between a metal and a base is shown as:

-$\mathrm{Metal} + \mathrm{Base}\to \mathrm{Salt} + {\mathrm{H}}_2$

$\mathrm{Example}-\mathrm{Zn} + 2\mathrm{NaOH}\to {\mathrm{Na}}_2{\mathrm{ZnO}}_2 + {\mathrm{H}}_2$

Hydrogen gas is produced when a base reacts with a metal.  

When a candle is brought near the test tube mouth it burns with a pop sound. This a test for hydrogen gas.

When a base reacts by responding with a metal, hydrogen gas is delivered at the point where energy from the electron is expended as potential energy. When a consuming light is held close to the test tube's mouth, it ignites with a solid snap.

Thus, the base reacts with metals and hydrogen gas is produced.

Hence, the answer is hydrogen.