Thanks Dan, please see below.
For any external, it depends on where it is installed. If you are using the Pd Document Dir, then Gem is probably installed to ~/Documents/Pd/externals. You should then find the folder in the browser via externals -> Gem or something similar. This works because ~/Documents/Pd/externals is added to the search paths by default if you use the Doc Dir.
I our case the default Deken installation path on Windows was used (i think, haven't got the machine nearby) yet the docs didn't show up in the Browser.