These lists will make it a little easier to get started. They are arranged as general to GitLab, specific to Strategic Marketing, then a few more recommendations and some just for fun, before sharing a few Slack pro-tips.
local channels: for your area or one you're visiting, hit command-K or 'Jump to…' in the upper left of the UI and search for 'loc_'. From #loc_nyc to #loc_pnw, and from loc_india to loc_berlin there are at least 75 local channels.
use stars: on the upper left of the UI, star the channels and direct message threads you use most
monitor keywords: on the upper left of the UI, hit the bell icon called 'Notifications' on hover, and at the bottom of the context menu go to 'Your notifications preferences…'. Scroll down to 'My keywords' and add as many as you like, comma-separated. Keywords some of use include 'PMM' and 'product marketing'.
tailor notifications: on the upper left of the UI, hit the bell icon called 'Notifications' on hover, and at the bottom of the context menu go to 'Your notifications preferences…'. Tweak to your heart's content, and don't miss the channel-specific notifications at the bottom of 'Your preferences for GitLab'.
#slack-power-users: of course! a channel for our friendly neighborhood power users. Join at your own risk.