I am an engineer, a consultant, or fresh out of an MBA. How do I actually switch into GTM?

Your background is not a handicap, it is your entry ticket, but only if you aim at the door it opens. Engineers should look at sales engineering, solutions architecture or GTM engineering, where credibility with a technical buyer is the job and you are competing against people who cannot read the docs. Consultants already do discovery, stakeholder mapping and business cases, which is most of enterprise selling, so aim at mid market or enterprise AE rather than starting at SDR. MBAs get pushed toward product marketing and strategy roles, which is fine, but the fastest learning is still on a quota. Whichever door you use, do the work before you interview: pick ten companies, research their buyers properly, and turn up with a written outreach plan or a teardown of their positioning. That single artefact beats any amount of explaining why you are passionate about sales.