Re Gitter: It integrates with the repository (eg. pasting commit hashes, bug numbers etc creates a link). It also has github signin. These things are really nice.
Re IRC: I absolutely hate Slack, but IRC holds no candle to Discord. Scrollback, search, highlight management, moderation tools, usable permission system, voice support, video support, low barrier of entry, online permanence, excellent file upload/image support, account-based identities, role customization and role-based permissions, support for profiles, drop-in group chat, markdown support with syntax highlight, multiline messages... am I done yet?
Re IRC: I absolutely hate Slack, but IRC holds no candle to Discord. Scrollback, search, highlight management, moderation tools, usable permission system, voice support, video support, low barrier of entry, online permanence, excellent file upload/image support, account-based identities, role customization and role-based permissions, support for profiles, drop-in group chat, markdown support with syntax highlight, multiline messages... am I done yet?