It's using Gradio, which is what auto1111 also uses. Both of these are pretty heavy modifications/extensions that do a lot to push Gradio to it's limits, but that's package being used in both. Note, it also has an api (checkout the --api flag I believe), and depending on what you want to do there's various UIs that can hook into the Text Gen Web UI (oobabooga) API in various ways.