definition of gclid by Google:
"Google Click ID (GCLID) is a parameter passed in the URL with ad clicks, to identify the campaign and other attributes of the click associated with the ad for ad tracking and campaign attribution."
The same applies for FB and TikTok (and others platforms using click ids), meaning if you send this click id value back to the platform (via FB CAPI or other S2S solution), they will be able to attribute that event to the correct campaign/ad and if not, conversion will not be attributed.
So i will say it is important, although it is not required by the documentation (because you can have traffic without fbclid).
But if you have it, be sure to send it and you will be able to create audience without sending email or phone number for that traffic.
external_id - unique user identifier, should not be your actual user id or any other user personal data and it should be the same on every event. It just informs Facebook / Meta that those events are done by the same user.
Here is text explaining in detail what are click ids (including fbclid) and why they are important:
https://floyk.com/en/post/how-does-fb-google-tiktok-click-ids-works-and-why-is-important