Epic and GOG don't make an app for Linux. Heroic Games Launcher is a free app that signs in to both and plays your games.
- Open Discover, the app store: its icon is in the dock. Or open Search (the Windows key), type Discover and tap it. Not there at all? See Add the app store.
- Search for Heroic Games Launcher, click it, then Install.
- Click Launch, and sign in to Epic, GOG or both. Your library is there.