Gene regulation is the process by which genes are turned on and off, which means that this is the process that defines which genes will be expressed. Differentiation is the process by which unspecialized cells acquire individual characteristics, reaching a specialized form and function.
It is through gene regulation that some genes are expressed and others are not, causing the cell to differentiate. This process is different for different types of mature cells, resulting in different specialized cells coming from the same unspecialized cell.