How to sync 30 days of mail in the mail app? I find managing multiple browser windows on OSX a real pain with no solution, so I decided to get rid of some tabs. 

I loaded up gmail into the OSX Mail client, but it wants to sync my entire account. Is it possible to sync only new stuff? Basically I want it to only sync the last 30 days and not everything...

It's trying to sync about 6gb worth of mail...
You can't do this in the OS X Mail client. Mail.app will download all your emails available on the server.

The workaround is to make the change on the server. You mention that you have Gmail, therefore you can go to Gmail's IMAP settings and restrict the amount of emails available (for example, the most recent 1000).



Also, you can exclude labels from being visible to IMAP, therefore Mail won't download the contents.