I started my current gap year research position a week after I submitted my AMCAS, but opted not to include my new job in my primary application as 1) I didn't have much to write about having not started the job yet and having only a general idea of what it would actually entail and 2) while the AMCAS allows you to now include "future" hours, I already had 15 other activities listed and didn't feel the need to bump my other activities. Whether you'll want to include it in your own AMCAS is a personal decision. Either way, however, you'll be able to update most(*) schools throughout the cycle with whatever you end up doing.
I updated all of my schools with information about my job and the productivity from it as the cycle continued. I spaced out the timing of this update depending on whether I had heard back from schools regarding interviews. For the schools I had yet to hear back regarding an interview, I sent the updates in mid-October and received invites from 3 of the 7 schools within a week of doing so. For the schools I had already heard back, I sent essentially the same update (personalized only with a short blurb addressing each school's specific fit/appeal) about a week after attending each interview. What I had been up to since graduation actually became a point of conversation at several of my interviews and was a nice way to add completely new information to the interview (which, I think, my interviewers often found more refreshing than the typical "I read you did X; tell me more"), so I'd say it worked out pretty well.
(*) Some schools won't accept updates to your file prior to extending an interview invitation, and others are happy to accept them at any point. Look at a few school-specific threads to see where the majority of your intended school list falls. That might be a factor in deciding whether to list your prospective job/volunteering work in your AMCAS.