The in-browser demo is very cool! It's not clear from the linked page, but the GitHub repo[0] includes links to sample tile datasets that can be used for the demo.
To the original authors, really needs a default tileset for the demo, with maybe a link to the files. An an progress/upload bar for when you're trying to upload a .zip. Tried, and appeared to stall out for minutes on upload with no response.
[0] https://github.com/zengyf131/gswt_renderer