You might consider allowing the gauge to expand with the window size. This makes the gauge a lot more flexible. When drawing a gauge it is useful to get a general coordinate drawing on screen that you can check your gauge drawing with. Both cartesian coordinates and radial coordinates are useful to check your drawing. There is a general layout drawing in the following.
You can use set sizes if you want to also. To keep your drawing area window size a set size check your vexpand and hexpand properties. Make sure they are false. Try using a GtkGrid instead of a GtkBox. Put the drawing area in a scrolled window and put that in the grid.
I have done some work drawing gauges and have a couple packaged as widgets. There are also some drawings of clocks, gauges, gems and gears in the above github cairo_drawings folder. Some resize as circles and some as ellipses. They might be helpful getting something that you can test a gauge drawing with.