guh is a lightweight client mod focused on simplicity and performance.
As of right now, guh is in very early stages of development.

See the installation page of the guh Wiki.

You can't. We internally discussed about supporting all Discord channels and even ran an experiment about it, but that didn't give great results. guh was not able to run in some scenarios due to the huge internal differences and made the whole client unusable (crash on start).

Long story short... yes. guh is against the Discord Terms of Service — but, you should keep reading:

As of right now, Discord is not going out of their way to detect client mods or ban client mod users. On top of that, guh does not make any manual HTTP requests unlike certain client mods / plugins, so your client's user agent is the same as a legitimate client. Meaning, Discord doesn't detect a client mod like guh. They can go out of their way to start detecting it, but they don't.

Hypothetically speaking - even if they somehow did detect guh, users are very unlikely to be banned on sight. It doesn't make sense for Discord to start banning a substantial part of it's userbase (client mod users) without any kind of warning. Not to mention it is mandatory for guh plugins to be fully API-compliant and ethical, implying guh users can't be banned for indirect ToS violations (e.g. selfbotting).