A byte is a measure of the storage capacity of computer memory. A hertz is a measure of a recurring cycle - the number of cycles per second the computer can handle. One does not 'contain' the other....
None. Megahertz (MhZ) is a measure of frequency/wavelength and gigabyte (GB) is a measure of capacity. Perhaps you were wanting to ask how many megabytes are in a gigabyte? That answer is 1000.