The element with an atomic mass of approximately 20 that exists as a gas at room temperature is Neon (Ne).

Here's why:

* Atomic Mass: Neon has an atomic mass of approximately 20.18 atomic mass units (amu).

* Gaseous State: Neon is a noble gas, meaning it is very unreactive and exists as a monatomic gas at room temperature.

Let me know if you have any other questions!