Streak's add-on for Gmail can be managed from your Gmail Settings, under the Add-ons tab.
If you're a G Suite user, you may need your team admin to remove the add-on for you.
- Open your Gmail Settings and select the Add-ons tab
- In the left column, click the Manage link under Installed add-ons
- Click the three-dot More menu and select Remove from the two options.
Removing the add-on here will remove it from all devices.