Found these: 
https://www.behance.net/gallery/58071711...ont-family
No Universal Fix  I have seen some similar reports. Fonts ( otf fonts?) working in other applications but not Gimp. Listed like your example.
I do not know what is wrong, looks like a naming convention in the fonts file (or maybe not)
A bit tedious but the otf font(s) name(s) can be 'adjusted' and resaved as a ttf using Fontforge.
In Windows, always a problem if the font(s) was initially installed system wide.  Try and remove what you installed.
For that particular font the converted ttf's attached. 
Unzip and put the files in C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\fonts
In Win10 / Gimp 2.10.12 looks like this. 
https://i.imgur.com/CcNopVj.jpg