Open the Calendar app on your phone.
Tap the menu icon (☰) or three dots, then go to Settings.
Tap Manage accounts or Accounts.
Make sure your Google account is listed and syncing is enabled.
If it's listed but calendar sync is off, tap the account and turn "Sync Calendar" on.
Go to Settings on your Android phone.
Tap Accounts or Passwords & Accounts (varies by phone).
Tap Add account > Choose Google.
Sign in with your Gmail account and complete any verification steps.
After it's added, make sure Calendar sync is enabled.
Go to Settings > Accounts > [Your Google account] > Account sync.
Make sure Calendar is toggled on.
Open the Calendar app, and your Google Calendar events should appear and stay in sync.