After pasting it in the channel, it'll float. You just need to go to the layers window and "Anchor" the selection (either through the anchor icon or from the context menu IIRC).
When you anchor it, it stops floating and "falls" on the place you pasted it in the first place. Whenever you paste things in Gimp, you have the option of "anchoring" it to the layer you pasted it or, if you create a new layer after pasting (i.e., Ctrl+Shift+N), to have it as a whole new layer.
I hope that helps.