As far as your current bid, if they have accepted the bid, then yes you are legally and contractually obligated. If you just SUBMITTED your bid and it has not yet been acepted, then you can pull out of it at any time by giving the proper notice (as your agent).
I have had some experience with GNND. It is a great program if you can take advantage of it, but it is difficult to find something. I wrote an article on the GNND program on my blog here: http://brianrayl.com/blog/teachers-fire-fighters-paramedics-and-police-more-financing-options-to-buy-a-new-home/
The selection is a "lottery" style selection where all people who put a bid in and qualify for GNND get put into a drawing and the one they pick out of a hat wins the bid. The best thing about the GNND program is that you only have to put $100 down if you are using FHA financing. Oh, and the fact that you only pay 50% of the asking price (you make a full priced offer on your bid, but then sign a promisary note saying that you will live there as your primary home for 3 years and they will waive the other half of the price).
If you can't find anything through GNND, then you may still qualify for special financing through certain banks. do a search on my website for what ever profession you are. I have a lot of special deals on my site through lenders that I use.
Best of luck to you!