What's responsible for preventing the gas giants from collapsing into a star?

Gas giants do not have enough mass that holds much gravitation force to ensure collapsing into a star. The gravitation pull is not strong enough to cause the fusion of hydrogen atoms. These gas giants, such as Jupiter, therefore, do not become stars.

They spin very quickly on their axes.

Explanation:

The gas giants spin very quickly, creating a strong centrifugal force. This strong centrifugal force prevents them from collapsing under gravity.