If you notice above your email accounts, there is a combined account option, click on that and you will see all your email together. They will be color coded by account (thin color line on left side). It is my understanding that if you want your gmail in with the combined accounts then you have to manually add it like a pop account. I have 3 pop accounts that I view this way.
Like jones said, you could have them forwarded to your gmail account. I have one pop account that does that but it seems like gmail doesn't check it very often.
Hope that helps
Chuck