When is it too late to submit an event for this year? I'm debating about trying to GM a few games and am curious how the process for submitting an event works. Can I submit an event for any game I want? I assume it's not guaranteed to be accepted.

Nope, not too late.  You sign in to your account and on the left hand side you'll see a link for "submit an event."  Just click it and follow the instructions.  It's also highly recommended you read the event organizer policy ( https://files.gencon.com/genconeventhostpolicy.pdf ), even if you're not going for reimbursement for anything - it tells you about ticketing, cancellation process, no-shows, etc.

Keep in mind that events submitted during early event registration are placed first before anything submitted now, so prime spots are probably going to be tough to get.  If you can be flexible with your times you're much more likely to get placed.  If you can give us some idea of what you want to run, you can probably get good advice here.