Are you sure you understand that printers requirements?
https://www.createspace.com/Products/Book/CoverPDF.jsp
...quote..Images may be CMYK or RGB color. All images should be sized at 100%, flattened to one layer and placed in your document at a minimum resolution of 300 DPI..
An image (for one face) might be 5" x 8" which @ 300 ppi (aka dpi) is 1500 x 2400 pixels
Then there are 2 faces + the spine + trim and bleed allowances. What are the dimensions in pixels of your image.
Truthfully Gimp is not the best tool for creating PDF's to send to a printer.
Use Scribus https://www.scribus.net/
Make your graphic with Gimp but if possible add any text in Scribus It remains a vector and will be crisper.
Reminder about opening PDF in Gimp. The default resolution in the open dialogue is 100 ppi If your PDF is 300 ppi change the resolution to 300 otherwise it will look small
https://www.createspace.com/Products/Book/CoverPDF.jsp
...quote..Images may be CMYK or RGB color. All images should be sized at 100%, flattened to one layer and placed in your document at a minimum resolution of 300 DPI..
An image (for one face) might be 5" x 8" which @ 300 ppi (aka dpi) is 1500 x 2400 pixels
Then there are 2 faces + the spine + trim and bleed allowances. What are the dimensions in pixels of your image.
Truthfully Gimp is not the best tool for creating PDF's to send to a printer.
Use Scribus https://www.scribus.net/
Make your graphic with Gimp but if possible add any text in Scribus It remains a vector and will be crisper.
Reminder about opening PDF in Gimp. The default resolution in the open dialogue is 100 ppi If your PDF is 300 ppi change the resolution to 300 otherwise it will look small