Welcome aboard,
@Guildme
1. GAD is our site owner's username, not a comment on the GAD acoustics of recent memory. Yup, your guitar is definitely not a Guild GAD.
2. The published serial number lists are wrong or incomplete for different eras. We do try to point people in the right direction to get answers for specific cases where guitars do not fit in the covered ranges. In your case, your D-55 was built as GGJaguar suggested.
3. Most of the members here do not have access to detailed information about guitars that extend beyond the published lists, but fronobulax is mentioning user Hans Moust as a published authority who has records available to him. You can contact him here, but I would not expect him to give you generic information; i.e., specific guitars will get specific answers if possible.
4. If you are interested, Hans' book (which we call the bible, see our FAQ threads) is a fantastic history of Guild Guitars but only covers up to 1977. Details about newer guitars are not available
Again, welcome aboard! I hope that helps answer your questions.