Just sent you a PM. I've said it elsewhere, and I'll say it again, I think GFD AO is the single best single role EMT job in the entire county. 100% 911, no IFT (unless you count the occasional emergent STEMI or Trauma center hospital transfer), all station based with no street corner posting, pay is better than some private company paramedics, you can even (if you meet the requirements) be sponsored through Paramedic school or a private fire academy). To get all those benefits, you have to go through a hiring process that is pretty much mirrored on a Firefighter Recruit hiring process (in other words, it's not apply, test, interview, and start all within a few weeks). There's a full formal written test, then a formal Oral Interview panel, and a full background investigation, complete with Polygraph and topped off with a Chiefs Interview. The whole thing takes ~6-9 months give or take.
As far as prepping for the written test specifically, there's your usual Reading Comprehension, Mathematics, and Mechanical Comprehension with EMT Basic questions. Nothing crazy or off the wall, very similar to a Firefighter Recruit written test, but more tailored for EMT. Definitely go ahead and brush up for sure, but don't stress about it. You're already an EMT, I presume you already took and passed the NREMT, that was more difficult. Get a good nights sleep, plan to arrive plenty early (you don't want to have to explain to a Chief why he should let you in the test if you're late, and there WILL be chief officers present). Dress professionally, I personally recommend suit and tie, though you'll be fine with business casual. Just don't be THAT guy showing up in board shorts and flip flops. Otherwise follow all directions in the letter they give you regarding what to bring and what not to bring.