I have never tried playing Gaudete (16th century), but it's pretty syncopated, so you'll have to be nimble. Plus there are two parts, which are significantly different from each other. Most versions of Gaudete are sung, but here's an instrumental where the melody is fairly easy to hear:
If you need sheet music, maybe one of these images may be of help for the version you want to play.
It suggests to me like it has enough minor notes that it'd be best played in either Aeolian or Dorian mode on the dulcimer.
Hobgoblin and Song of the Wood are just two companies that make accessible plucked psalteries. You can find other nice ones on Etsy. You may have to decide whether you want a diatonic or a chromatic one. Also the shape is varied depending on the maker, including 'hog-nosed' shaped psalteries.
Hope this helps.
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