I consider the knitting gauge “mildly essential” but, I want to be clear that I am NOT saying that checking gauge is unimportant. Gauge is especially important when it comes to garments, but I'll make a post about checking gauge in the near future. I purchased one of those beginning knitting kits when I first learned to knit and I think that it came with a knitting gauge that I never used. It was a long time before I knit anything where gauge really mattered. The reason I say that a knitting gauge is mildly essential is that you can use a standard ruler to check gauge.
The pictured gauge is from my Boye set of interchangeable needles. The 2-inch vertical and horizontal windows on the left edge are what I use to check the rows and stitches per inch, also known as gauge. I use the needle gauge on a regular basis since sizes aren't marked on my interchangeable set. It also has a 6-inch ruler across the top for small measurements. Basically, I use this for a lot more than checking gauge. You can buy these alone and they are often packaged with kits. It's also worth looking into some unique and handcrafted gauges since it is a tool you can buy once and keep forever.
Do you use a knitting gauge or ruler to check gauge?