Hi there Liamo!
You have several questions interwoven here, so correct me if I'm wrong, but it sounds like you are asking if you can make substitutions for liquid black and black gesso.
Before continuing, I must reemphasize Jin's point that Black Gesso and Liquid Black are NOT interchangeable. One can not be used in place of the other for their intended purposes.
The intended purpose of Black Gesso is to change the base color of your canvas to black. It is an Acrylic-Based primer that must be allowed to dry completely before starting a painting. When priming your canvas, you can substitute black gesso with any acrylic black paint, as far as I know. If I am wrong here, someone please correct me.
The intended purpose of Liquid Black is to provide a wet surface on which to paint. It is an Oil-Based medium that must remain wet throughout the painting in order to do its job. Regarding substitutions for Liquid Black, you could make your own by mixing a black OIL paint (Lamp Black) with linseed oil in about 50/50 ratio. I would make the mixture first, before applying it, rather than mixing on the canvas. This makes it easier to control the ratio.
Last little tip about using either Liquid Black, Clear, or White, is to use as little as possible by applying it evenly, and then do a pass with either a dry brush or a paper towel to remove excess.