1977 Robert Mize
Instruments- discuss specific features, luthiers, instrument problems & questions
I'm relatively new to dulcimers also, I got my first one in the spring of '23. I'll put my two cents in and tell you some basic info, I'm sure you'll get more posts from more experienced player soon.
From viewing the photo of the pamphlet, I think that what the maker is telling you is that if you go to s music store to buy ball end guitar strings you should buy 3 high E strings and a G and that is what he originally strung the dulcimer with. He's providing this info so you can get the right thickness of strings for the instrument. I use light gauge strings on my guitars and the high E is .12 and the G is .24. You won't have to do that, though, since you bought a set of dulcimer strings.
the VSL (Vibrating String Length: the distance form the bridge( toward the bottom of the dulcimer) to the nut (at the top of the neck) is a factor in determining the "best" exact gauge for the dulcimer, though there's plenty of leeway in that regard. The McSpadden DAdd string set you got is for a VSL of 28 1/2". If the VSL of your instrument is near that length, they should be good enough. You wouldn't tune the strings to Geee or Egee, you'd tune them to DAdd. There are many more tunings possible as you will discover, I hope this helps you a little...