You are using some sort of Windows, do not know version. Printing from Gimp in Windows is generally broken. The old Gimp 2.6 might be better, Gimp 2.8 has problems.
The bottom line is do not use Gimp for printing. Save your work as a Gimp .xcf file but when complete Export the image as a .png or a .tif
Then, do what you are already doing and use some other application for printing, IrfanView or personally I like XnViewMP. Up to you.
There was a recent post using Win10 + Epson where Gimp used Print-to-the-Microsoft PDF printer then the Win10 default image viewer uses that & prints to Epson.
You can correctly set up a paper size in the MS PDF printer.
The bottom line is do not use Gimp for printing. Save your work as a Gimp .xcf file but when complete Export the image as a .png or a .tif
Then, do what you are already doing and use some other application for printing, IrfanView or personally I like XnViewMP. Up to you.
There was a recent post using Win10 + Epson where Gimp used Print-to-the-Microsoft PDF printer then the Win10 default image viewer uses that & prints to Epson.
You can correctly set up a paper size in the MS PDF printer.