Some e-mails are more important than others. As a freelance writer/web designer/translator I often get time critical messages. I want to get an email notification with a sound alarm and a vibration for those, but not for all my mail (my phone would be abuzz continuously).
So I found this useful article on how-to geek. It is free and really simple to set up in gmail. You basically add a filter label and set your notifications only for that label in Android (the label needs to be created on a pc though).
Gmail notification is just a part of the puzzle for freelancers, especially those who want to be the first to respond to job offers.