Not 100% clear from your description.
If you have successfully removed the background and there is an alpha channel, then the background shows in Gimp as a checker pattern. No alpha channel then no checker pattern, it will show the current BG colour.
If you use some other viewer then it might not show the transparency, the background might be white or sometimes black.
You do not need another layer under the image. To retain the transparency save that single layer as a Gimp .xcf file or export as a .png file.
Problems further down the line?
Beginners often have problems understanding copy / paste in Gimp (that floating selection temporary layer) If you get one of those, either anchor it (merges down) or better still use Layer -> To New Layer
A different way is Open the background image, then Open-as-layers your previously Saved .xcf
If you have successfully removed the background and there is an alpha channel, then the background shows in Gimp as a checker pattern. No alpha channel then no checker pattern, it will show the current BG colour.
If you use some other viewer then it might not show the transparency, the background might be white or sometimes black.
You do not need another layer under the image. To retain the transparency save that single layer as a Gimp .xcf file or export as a .png file.
Problems further down the line?
Beginners often have problems understanding copy / paste in Gimp (that floating selection temporary layer) If you get one of those, either anchor it (merges down) or better still use Layer -> To New Layer
A different way is Open the background image, then Open-as-layers your previously Saved .xcf