There is a spot in my map where I need to have low gravity, since the game limits how long I can wall run for. I found a post here https://ugx-mods.com/forum/index.php/topic,7767.15.html , but the code does not work anymore.

What do you mean the code does not work anymore? What is the issue with the code exactly?

That tutorial is for World at War, not Black Ops 3. You would have to change a lot of the code functions since many of them is not available(or has different names) in black ops 3.

There is a gravity KVP already available in Radiant on the worldspawn, but I don't know how to make it for only 1 area, or if it even works.

I haven't tried yet, but there is a function for this in bo3:

In the docs you will see it like this:

void <player> SetPlayerGravity(<gravity>)

[MANDATORY] <gravity> The gravity to set
CATEGORY:
CLIENT/SERVER: Server
SUMMARY: Sets the gravity override value for the player.
EXAMPLE: player SetPlayerGravity( 600 )

It means to call this function on the player you want to change the gravity on and returns nothing. It also states that it is server (gsc).

There are several functions related to it. I've also had some difficulty realizing the grand differences between waw and bo3, as it is hard to unlearn the limitations, but a lot of things can be done now, and are done completely differently. Check out the functions, and maybe use an autocomplete tool when script to see the available functions, then research them to see if they are what you want, then try them and see if they worked!

Thanks mate, this is really appreciated!