One of the most important things before you start drawing icons is to lay out the program windows for your individual comfort.
Because GIMP is not an MDI application, managing GIMP's windows is solely a window manager's task. Personally I'm very happy with Sawfish WM, which includes a feature coming originally from MacOS - roll-up windows. But sawfish goes overboard by enabling you to roll-down windows upon moving the mouse cursor above the rolled-up window. What sounds a bit compicated is actually very handy and useful for small screens, like a notebook LCD. The rolled-up windows don't take any space and the content of the window is easily accessible just by moving the mouse cursor over.
Now we need to create a new artboard with Ctrl+N. Standard size of a Gnome icon is 48x48. Because working on such a tiny pixmap requires a lot of sense for detail, we'll zoom in to be able to work at a pixel level. I usually work at 8:1 (View->Zoom->8:1). But in the microcosmos of the 48x48 grid you will quickly lose an overview of the artwork. Therefore we'll use GIMP's ability to open multiple views of the same artwork. Choose View->New view from the image context menu. Use a 1:1 zoom on this. That way you can always check the result even if you're going to paint at 8:1.
Make sure to turn off the selection decorations on the 1:1 window with View->Toggle selection (Ctrl+T).