EDIT: i can already do this by using the Gap function.
It’s still almost impossible to design for fluid widths in Figma
But with the new Grid Beta, it is now possible to set up for example a 12 col grid system , see my screenshot:
https://prnt.sc/fetKFckcQpT4
Basically I set the colums to 1fr, and i set the gutter to my design in pixels. This is how most grids work on a webpage. Now if i rescale horizontally, it keeps the gutter, and scales the grid like in real-life.
But on smaller screens the gutter usually need to be smaller, and it would be extremely tedious to manually replace 12 gutters for EACH and every frame i use this on a webpage (potentially hundreds)
So if we could use variables to the colums in Grid Beta, that would be just one click solution and we almost got a working fluid system in Figma.
(There was this old approach using Layout Guides, but that’s not compatible with auto-layout which is a must-have for any day to day work.)