If you purchase a Google Drive storage plan with a Google Apps Account, your Gmail storage amounts will not change. This is because storage is shared only between Drive and G+ Photos, not Gmail. If you purchased a Google Drive storage plan with a consumer @gmail account, your Gmail storage will be upgraded to 25GB of total storage, only. This is a courtesy upgrade when choosing from any of our plans; the storage plan you choose will not be for Gmail, only for Drive and G+ Photos. For example, if you purchased a new 100GB plan you'll get 100GB shared across Drive and Google+ Photos, with 25GB total for Gmail.
