Here's an iPhone tip / trick from MacWorld. The iPhone's mail client has no spam filter at all. Here's a way to filter spam through Google's Gmail before it hits your iPhone mailbox. We've actually done this for our other email accounts, so we can confirm it does indeed work.
It is rather a slightly cumbersome process but a necessary one if you want to keep spam on your iPhone to a minimum. Check out this article on how to kill spam in your iPhone's email client here.

but will all my email on my iphone say that they are emails from gmail? and if i send an email from my iphone will it be from my gmail?