A GED and EMT certificate is most likely all you'll need for an EMT-Basic job. It really depends on where you're located, but you have a good chance of getting an EMT job. You just need to be persistent and determined. Good luck!
Completely agree with Matt here. We can talk a lot of generalities here. And generally speaking, this is a non-factor for you. Organizations that actually hire EMT-Bs are looking mostly only for warm bodies with a patch. If they had some significant concerns over education, they wouldn't be hiring an EMT-B in the first place, would they? It's just a three week first aid course taught at the 8th grade level. And there is no evidence that I am aware of that shows GED holders are any less intelligent than their HS grad counterparts, so again, it's a non-issue.
The question is not whether or not you can get an EMT job. The question is whether or not you can get a real EMS job, which I would hope is your ultimate goal. And, as Matt points out, where you are located is the primary factor that will determine this. If you live someplace where there are no EMS jobs for basics (because EMS is run by fire departments or an all-paramedic agency), then it doesn't matter how you did in high school, you aren't getting a job.
That, of course, leads to the following question; do you plan to become a paramedic? There is no career as an EMT. It's just a temp job. And where you may ultimately run into problems is getting into paramedic school. The better programs are either college based or require college prerequisites for admission. That means you're going to have to take some pretty serious tests over general knowledge to get accepted into the local junior college. If you are confident in your knowledge, then that won't be a problem, but just another hoop to jump through. But if you struggled in high school, and struggled to squeak through the GED exam, then this could end up knocking you out of the running. And if you are stuck at EMT for your entire career, you have no career. That said, if you are able to get into and pass paramedic school, you will definitely find a job.
Matt was right on the money when he mentioned your location. It makes all the difference. That is the key to whether or not you will ever even get an EMT job, much less a real EMS job. But if you are willing to move to where the jobs are, you won't have any trouble getting an EMT job of some sort, even with a GED. Good luck!