Is it an absolute must to use gesso when paining on canvas? ?

With acrylics, no, gesso isn't totally necessary. It's nice to use because it gives you a good surface to work on and helps keep the canvas sized, but forgoing it won't ruin anything. It's a different story with oils, where the paint can degrade the raw canvas, and a barrier such as gesso or hide glue is required.

Gesso is for stretching canvas but it also has some archival value so in the long run the paintings will not degenerate or shift as easily with time. It is really up to you if you want to use gesso or not.

I in particular do not like the white look that gesso creates so I have used different gel mediums instead, not only does it stretch the canvas keeping the raw fabric look but it also seals the fibers maintaining it's archival value.

...most people don't care about archival value because the fibers in paintings do not noticeably degenerate in their generation.

it depends on if you bought it stretched or you stretched it your self because they already put a coating of gesso on it when they stretch it but if you stretched it your self then it won't have it on, and it doesn't really matter if it doesn't because it just makes it easier, you can paint straight on to canvas with out it its just that the colour fades slightly.