Ok, thanks both for answers. Now I remembered I have Corel PhotoPaint that wes mentioned above 
I opened there a 300dpi JPG exported from CorelDraw, and surprise surprise, both Y and X dpi-s are 300. So, now I'm convinced. And I might continue using Corel PhotoPaint instead of Gimp since I have to be sure about the image quality. Besides, when exporting a JPG from Gimp, there is a checkbox "Optimize" under Advanced Settings arrow, which is "hidden" and checked. So the quality will be optimized what I don't like. But now I know the checkbox.
I opened there a 300dpi JPG exported from CorelDraw, and surprise surprise, both Y and X dpi-s are 300. So, now I'm convinced. And I might continue using Corel PhotoPaint instead of Gimp since I have to be sure about the image quality. Besides, when exporting a JPG from Gimp, there is a checkbox "Optimize" under Advanced Settings arrow, which is "hidden" and checked. So the quality will be optimized what I don't like. But now I know the checkbox.