>Lost in new Gimp

[rich2005]

I remember.   Are you using Gimp 2.10 now ?

Nice TR3 even though the steering wheel is on the wrong side

I think your crop is a bit too much, keep as much as possible at the bottom.
You will not get 300 x 300 without borders but that might be your new canvas
First the crop: Tools -> Transform Tools -> Crop (shift-c)   That will give an image 700+ x 576 pix ( the image height)

   

Then Image -> Scale Image Break the link between width and height Set the width to 300 pix.

   

If you want the image square then borders are required top and bottom.
Add an alpha channel Layer -> Transparency -> Add alpha channel
Increase the canvas size Image -> Canvas size Break the link between width and height. Increase height to 300 pix Center the image and Set a Fill with. see the first image here https://i.imgur.com/MP9TdOn.jpg

This does repair quite well using the add-on resynthesizer / heal transparency plugins - but that is another topic.

edit: nearly forgot the blur
Use the free select tool to make a selection. Then Filters -> Blur -> Gaussian Blur  Adjust the value to suit. Then Select -> None.