Back in the days of machine tool repair, we had a shop make our gibs for us. They would buy a slab of cast iron of a determine thickness, usually about two foot square and about 1" to 2" thick. In fact, they kept several thicknesses of cast iron plate on hand just for gib making. They had a smaller openside planer in house that they used for this so they didn't tie up the "beast" for real planning jobs. Took a part off tool and would rip off pieces of varying thicknesses with a taper that matched closely with the original. Once that was done, they set up a angle fixture that was a magnetic chuck and start working the angles until the gib shape was made. Next it went to the machine component the gib was made for and the master scrapers fitted the gib to the slide. Last, the gib was cut to the necessary length and gib screws fitted to the gib and slide.
Just from my own experience, if you use a surface grinder, you will have a problem with the gib warping on you. It takes lots of flipping from one side to opposite side, doing this many times taking only a thousandth or less to keep warpage to a minimum. I find it easier to set up on the mill with a fly cutter or inserted face mill that uses a high positive insert to mill the faces of the gib. You still have to leave material and take same amounts from opposite sides to get the warpage to go away. But with the face mill it tends not warp as bad as it will on a surface grinder. Making a gib out of steel is common, but I prefer cast iron. It's much easier to scrap than steel is. And don't use brass or bronze for a gib. Ken