If a gas boiler is leaking, you should not attempt to repair it yourself; gas boiler repairs should only ever be conducted by Gas Safe Registered engineers.
A leaking boiler is actually quite a serious problem, and indicates that an internal component such as a seal or valve is broken. The leak can lead to corrosion and rust, or can cause electric components within the boiler to short circuit. It could even cause problems throughout your house, including structural damage. As such, leaking boiler repairs should be conducted as quickly as possible.
Determining the cause of a boiler leak can be a challenge and requires finding out exactly where the leak is originating. For example, is your boiler leaking water from the bottom or is the boiler leaking water from the inside?