There's another way to think about gauge, and have it simplify your life a little. But you're still gonna have to knit a gauge swatch. And it only works when you're doing your own designs or seriously altering an existing pattern. But still. Here we go.
For our example, we're going to use my current project, and I'll just put up a nice photo of the gauge swatch again so we have something to visualize:
So here's our swatch. We could do the usual, measure stitches per inch/centimeter, and figure from there, but in patterned knits like this it means all kinds of odd half-repeats and jiggering around to center things. Lots of math and frustration involved. Instead, measure the pattern repeat. Use IT to figure gauge. For instance:
This pattern repeat is 4.9 inches. I need a 50 inch sweater, so ten pattern repeats would get me a 49 inch sweater and I could probably fix the other inch in blocking. (Yes, this is very convenient. So conveinient I checked the measurements and math five times.) Five pattern repeats for the back, two and a half for each front. A pattern repeat is 18 stitches, so ten pattern repeats is 180 stitches. I cast on 180 stitches and started knitting. I'm still not sure exactly what the stitches per inch figure is.
If I wanted, say, a 52 inch sweater, instead of wedging in odd fragments of pattern repeats, I could still use ten pattern repeats, and cast on a few extra stitches at each side, working them either in a solid stripe or a checkerboard, or whatever, and still have whole pattern repeats to work with.
Again, this only works for design or serious pattern overhaul, but when you can use it, it's very helpful. (This works EXTREMELY well for cable knits, in particular, because it's almost impossible to do half a cable, or a third of one, or whatever.)