If you are using GMail's Outgoing server as Ugly
is doing, there is no real need to set a folder. GMail will store the outgoing mail in its own sent items folder. In fact unless you want a duplicate of every sent item it is best to to tell Outlook not to save the sent mail.
That is a major annoyance for me with Outlook 2013. If you are setting up a GMail account no matter what Outgoing server you use and whether you tell Outlook to save sent items, it will take the check off the option thinking you really didn't mean that. You don't notice it hasn't saved any sent mail until it is too late and go back and check the setting to find out Outlook has changed the option without your knowledge. This would only make sense if you were using the GMail Outgoing server. In any event they shouldn't change an option without telling you they are doing so. They also did away with the ability to specify Sent and Deleted items locations although you may be able to map that with your IMAP account.