At 2000x2000, 300 ppi, the real print size is 6.7".
Gimp is an editor, when you zoom in it shows bigger pixels, because you want to see the actual pixels (so, at 400% zoom, a pixel is displayed as a 4x4 square). Photos is a browser, when you zoom in it shows a bigger image, and it scales up the image by interpolating pixels, so the lines are not pixelated but become blurry.
If you need a bigger image in Gimp, you can scale up the image (Image > Scale image, Scale tool, etc...) and unless the interpolation methd is set to None (typically, NoHalo in Gimp 2.10) then you will see something similar to what Photos does.
For some context about what the image size means, see this.
Gimp is an editor, when you zoom in it shows bigger pixels, because you want to see the actual pixels (so, at 400% zoom, a pixel is displayed as a 4x4 square). Photos is a browser, when you zoom in it shows a bigger image, and it scales up the image by interpolating pixels, so the lines are not pixelated but become blurry.
If you need a bigger image in Gimp, you can scale up the image (Image > Scale image, Scale tool, etc...) and unless the interpolation methd is set to None (typically, NoHalo in Gimp 2.10) then you will see something similar to what Photos does.
For some context about what the image size means, see this.