Online publications like GIG can take more last minute advertising because there are no printer concerns, however, premium banner ad space may already be reserved. If you don't already have a booth to use in your booth space, you should be arranging for one now. GDC is a competitive conference and your competition will rat you out if you have booth paraphernalia spilling over the sides or the top of your space. And sometimes they don't just tattle to GDC reps, they may just knock it down. It's not nice, but it does happen.
Nothing is cool or creative once it's lying in a crumpled mass on the floor.
Avoid the conflict altogether by planning for and abiding by the dimensions of your booth space. Also, by the end of January, you should have your marketing material drafts ready for review. This will allow plenty of time for everyone to review and then review again, make changes, and then review again. If you want to have a little get together for your past and potential clients, you should be booking your bar or restaurant space now if not sooner. If you slacked off and didn't do what I said in January, then early February is the time to hustle your bustle.