Adapting from other graphics-manipulation software is not seamless. For example, if you opened an image file in MS Photo Editor or P'shop, and you wanted to make a new file with just a select portion, you'd use the rectangle select tool, select your selection, find the "Crop" command in the menus, and save. Very neat and intuitive. GIMP comes at the procedure with a slightly different terminology - select your selection, and then "Fit Canvas to Selection".
One of the things I couldn't figure out how to do, and there was a dearth of relevant tutorial material on the interwebs, was to paste two images into one file. I had postcard-size show flyers from one of the 2002 or 2003 Miami Music Festival parties that I wanted to scan both sides and save as a one-page image. I fiddled for hours with no real progress.
Another procedure that web designers would find familiar is picking a selection, 1 pixel wide by the full height of the image. (Or 1 px high by full width, same principle.) Nowhere in GIMP do I find a select-by-coordinates tool, and the "select" tool is extremely awkward on small selections. I ended up using the "crop" tool and trial-and-erroring my way to a 1px-wide selection to crop to.