Perhaps the most obvious way to zoom in and out of your files in GIMP is to use the various options in the View menu item. There you'll find a Zoom item that displays the current zoom level in parentheses and if you mouse over this, a new sub menu of zoom options is displayed. This offers a number of preset zoom options that you can select from as well the ability to zoom in and out in incremental steps.
While this is a straightforward and intuitive way to zoom your documents, it can be a little obstructive to a smooth work flow. However if you take a closer look at the sub-menu, you'll see that almost all of the entries have a keyboard shortcut listed next to them and using these can make your working style much more fluent. I personally get by with just using the + key to zoom in, the – key to zoom out and the combination of Shift+Ctrl+J to fit the image to the available space in the window.
You may be happy enough using keyboard shortcuts for zooming, but there are also some more options, including the Zoom Tool and the Navigation dialog.