You can use HTTP Basic Authentication with your account’s Api key by replacing the username with your Api key and the password with an arbitrary value:
curl -u api_key:foo https://contentgems.com/api/v1/...
You can find your account’s Api key in the ContentGems web app’s user profile page. Navigate there by signing in to the web app and selecting ‘Configure’ / ‘Your Profile’ in the top bar.
Make sure to keep your Api key confidential, just like your password.