Providing the image would be a plus or a screenshot with the layer's dialog.
When you said "I added to GIMP"... Is it a new image or you added it on an image already opened in GIMP?
For the moment just guessing what you did, may be your image has transparent border, thus when you scale the layer ➤ you keep these transparent border, keeping a "gap"
Possible solution: Select your layer you want to re-scale, go to the menu Layer ➤ Crop to Content (this action will remove a possible transparent border),
then now you can re-scale without gap.
Another possible solution: May be you are trying to scale the full image instead of a layer, go to the menu Image ➤ Scale Image
When you said "I added to GIMP"... Is it a new image or you added it on an image already opened in GIMP?
For the moment just guessing what you did, may be your image has transparent border, thus when you scale the layer ➤ you keep these transparent border, keeping a "gap"
Possible solution: Select your layer you want to re-scale, go to the menu Layer ➤ Crop to Content (this action will remove a possible transparent border),
then now you can re-scale without gap.
Another possible solution: May be you are trying to scale the full image instead of a layer, go to the menu Image ➤ Scale Image