There are a number of ways of handling an SPG. The simplest is to route the hose straight down, and clip the gauge to your BC at your left hip. This may not be easy, if your BC doesn't have a ring or loop or anything to clip to in that site, but most will have SOMETHING along the bottom hem, somewhere between your hip and your midline.
Another way to route it is under your arm, and clipped up to your left chest d-ring. This may make the gauge easy to see while you are diving, without having to unclip it. Most BCs have some kind of ring on the front of the shoulder so you can do this. You can also clip across your body to the right d-ring, if the hose is long enough, as people do with their consoles.
Both of those approaches have limitations, if you are going to go on to more complex diving, or use a long hose. The left chest d-ring approach doesn't work well with slung bottles, and the right chest one is difficult to manage with a long hose.
Since one generally checks gas only every five minutes or so, having to unclip, check and reclip really isn't that big of a problem, and the left hip approach has none of the listed limitations.