Change default mail application without configuring mail.app

You can configure Gmail as IMAP account type. That will sync your mail messages between multiple mail clients.

That leaves open your request to not retrieve all 3Gb of mail when first configuring the program. There I haven't got an answer only a possible workaround. Many mail clients limit their download sizes when they are on low bandwidth connections. You could try to limit your bandwidth, for exmample to 8 kbps, with these terminal commands when having Gmail configured to use secure imap:

$ sudo ipfw pipe 1 config bw 8kbit/s #create reduced bandwidth pipe
$ sudo ipfw add pipe 1 src-port imaps #sent traffic from imaps thru pipe

As Gmail is not offering an Exchange server, only ActiveSync compatible, you will never get an exact similar experience with Gmail.

Select the Sent folder for Gmail in the Mail.app sidebar (might be under a [Gmail] heading in Mail depending on how you set it up) then go to the Mailbox menu in the menu bar and look down near the bottom for the Use This Mailbox For menu.

Choose Sent in there and that should make the Gmail-based Sent folder the default place for Mail.app to save outgoing mail. It will also appear to move that [Gmail]/Sent folder out of the Gmail section and up to the top-most Mailboxes section below Inbox, etc.

After you do this you may notice you now have a section in the Mail sidebar for On My Mac folders, and a Sent folder in there. This is the folder that previously was containing all messages sent via Gmail but stored locally. Feel free to copy all the messages out of there and into the new default Sent folder (which will copy them all up to the Gmail server).

PS - This process works for Trash as well, so you can map the Gmail Trash to be used by Mail.app. You can also do it for Drafts but I've seen some weird behaviour where Gmail/Mail.app doesn't delete drafts after sending a message and you get a ton of them stacking up in Drafts, which is why I usually leave Drafts as a local-only folder.