Giga- is a prefix with a factor unit of 1 billion, or 1,000,000,000. To convert gigahertz to hertz, you must multiply by 1,000,000,000, since 1 gigahertz is the equivalent of 1,000,000,000 hertz.
For instance, if you have a value of 2.5 gigahertz, multiplying by 1,000,000,000 hertz per 1 gigahertz will give you a value of 2,500,000,000 hertz. Similarly, a value of 35 gigahertz is equivalent to 35,000,000,000 hertz.
You would use a value with the unit hertz when you are performing operations pertaining to a problem of frequency. The unit hertz is equal to the number of cycles per second.