I can't believe no-one makes an iOS app with built-in junk filter.
Junk filters on mail apps are less efficient as server-side junk filtering. With the latter, you don't waste bandwidth on crappy spam mails.
Gmail's spam filters are very advanced and i rarely get spam in my mailbox. I have a catch -all on my domain so the spam filter catches about 35-40K spams a month. Marking arrived messages as spam is as easy as putting them under the spam label.
Some users tho have a habit of dumping their emailadress everywhere. In my experience, that's where most spam and junk comes from for them. Also there are many "bad" websites and apps that sell your emailadress. I can identify them by adding a +somethimg when i sign up somewhere (myname+theirname@gmail.com) so i can filter it and recognize who sold my address.