ME TOO! i wish there was a fix, but no one has come up with one that actually works for me. i assume this problem is occuring across the board for gmail users. at least, ive read over 20 posts (here and other sites) with the same complaint. im sure google (or Apple) will release an update to fix this.
i hate to say it, but never had this problem on BB. it only downloaded new unread emails. the iPhone downloads every email ive ever had always keeping my inbox FULL. its annoying, but at least all new emails still make it to me.
In my experience, by default, the mail feature will keep pulling down the 50 most recent messages from your gmail account inbox, whether those are new or old messages. So if you keep deleting messages off from iPhone, it will retrieve OLDER messages just to present you 50 constant messages on the iPhone.
Go to iPhone Settings -> Mail to change the number of messages to be shown.
Otherwise clean out your gmail inbox directly.