I reralized after I posted that all of us who replied didn’t really answer the question:
“what are the charactaristics of a “Gothic” harp”
Here is a link that may help: http://www.eric-harps.de/historical-harps/gothic.htm
There are sound clips with and without brays of most of the models he makes along with a nice explantion of where he found inspiration for each one. There are other places where you can read more about the construction of harps and the music written for them in the 15th and 16th centuries, but this site is the only place I have found sound samples offered for instruments