It's best to use layer masks in GIMP for this kind of work. Then you can simply edit the mask to reveal or hide parts if you made a mistake. The beauty of using layer masks is that they are non-destructive, and so you edit the mask rather than the image itself.
Basically, make a selection of the object using any tools you want. If you are selecting the background, invert the selection afterwards so only the object is selected. Then right click the image layer in the Layers panel, and choose Add Layer Mask. In the dialog that opens, choose the option Initialize layer mask to: Selection
Once you have a layer mask, you can select it in the layers panel. Paint on it in white to reveal parts of the image, or in black to hide parts of the image, or in grey for semi-transparency.
Example demonstrating painting on the layer mask in white to reveal parts of the image: