When purchasing a DLC here on GOG, for a base game you already own, does that base game also have to be on GOG? I want to be sure before I purchase any DLC from GOG, for a base game on another Platform (of course I probably already know the answer, but I'm old).

You can't buy a DLC on GOG if you don't have the base game here already.

Yep, you have to own the game here to purchase the DLCs, it adds as a complement download to the base game.

What if you wanted to gift a DLC to a friend?

Makes no sense to have buy the game just to buy a DLC.

You can gift DLC. You can't redeem without the prereq base game.

I've done this to get DLC at a steep discount when the base game wasn't at a price i wanted yet. So I'd gift the DLC to my email, and wait for the main game to go low enough to buy it and redeem the DLC.

Years ago i'd buy games with deep discounts and gift to myself for future trading or gifts. (Recently going through them, i got a ton of them surprisingly...).

Regardless, complete with the DLC and base game however you need. There's generally enough sales/discounts you can get most games priced okay eventually as long as you don't HAVE to play/have it right away.