|Steve Gaynor (before he changed his hair!!!) presenting on Gone Home at Level Design In A Day 2015.|
Historically, many of the presenters have been game industry level designers -- but there have also been architects, indie designers, procedural level designers, programmers, artists, and more -- and the directors are committed towards a diverse and inclusive idea of level design. They even let ME give a talk once! Wow what were they thinking?!
The most common type of session here is a case study, where a speaker talks about specific levels in a game and analyzes some facet of the design or production, what went right and what went wrong, etc. There have also been broader talks about level design as a discipline, questioning how we generally think about level design or how we practice it. And sometimes there's talks that try to translate a "different" field, like architecture or writing, to level design. (This isn't to say your talk must fall into one of those buckets, but those are just the most common buckets.)
To submit your proposal for consideration, please use this web form. (Don't use the official GDC submission procedures.) You must submit by November 6th (in ~2.5 weeks) to be considered. Good luck!
(Addendum: traveling to GDC is expensive. Traditionally, GDC does not pay any travel grants or stipends to most speakers. If you work in the game industry, your employer is supposed to pay your way; if you're a student, your school or government hopefully has travel grants for you to participate. As a last resort, you may also want to try a GoFundMe to raise the funds.)