never use the gate, that for sure.
you can pre-book using loads of websites, but try to get one recommended. And make sure you get tickets, not a voucher to exchange for tickets at the gate.
where are you staying ?
there are cheap ticket booths all around I-drive.
I bought my multi-park passes before I left, I found it easier to budget that way. I did get tickets for NASA from a booth in a mall though.
beware if buying cheap ones though, that at Disney and Universal they ask you to sign the back of the ticket AND you need to scan a finger/thumb every time you enter. (you choose which finger the 1st time to use the tickets), so if you have someone else's ticket, you won't get in.