leenia, I think I may be able to help. Tuning a GDD dulcimer down to DAA isn't going to work without new strings. If you can post the vibrating string length of the dulcimer in DAA with strings of 12,14 and 22 gauge, then give me the vibrating string length of the GDD dulcimer, I have a chart that should be able to give you the equivalent string tension and therefore string gauges in whatever tuning you wish. Just let me know your preferred tuning.   

I have 2 other questions.

Do you sing to the dulcimer? If so, your preferred keys will dictate how you tune it. Even if you have a dulcimer capo, you still need to choose your most comfortable pitch for usual, uncapoed playing.

I played dulcimer (when I did, don't any more) in DAD, where D was the highest and lowest note. If your preferred tuning is DAA, please indicate which is your order of pitch (I'm guessing you have a D on the bottom, an A then another A at the same pitch, but in that case I wonder why you have different gauges for the 2 As; but if D is at the top I can't make sense of only 2 thou between the D and A below it).

Stower