How to crop a rectangle with rounded corners.

[rich2005]

There are a few ways: Relatively simple is (more-or-less the same as previous but no need to invert selection)

1) Select -> Select All
2) Use the Rectangular Select tool and click in the canvas to activate.
3) Enable Rounded corners in the tool options and adjust the slider
4) Edit -> Copy
5) Into your project file and Edit -> Paste.
6) Move into position and then Layer -> To New Layer

30 second example. https://i.imgur.com/XYqSArb.mp4

If you want to keep a copy of the image with rounded corners to use later then:
Edit -> Paste as -> New Imge and Export as a .png to keep the transparency.
Add to your project with File -> Open as Layers.