Hello and welcome,
For some reason you're here wondering if you can use this tool to visualize the internal structure of an index of yours.
It requires python3.6.*. You'll also have to install pageinspect (https://www.postgresql.org/docs/10/static/pageinspect.html) extension. If you want to use this tool for GiST indexes, you'll have to install gevel (http://www.sai.msu.su/~megera/wiki/Gevel) postgres extensions.
You can clone this and then run
python setup.py develop
The command line is then
python inspector/command.py --host <host> --port <port> --db <your_db> --user <youruser> --index <index_name> --path <your_path>
It will generate a wonderful html. The path is the path of the html file to be saved.
Right now it only works with BTree and GiST. I will add BRIN index eventually :)