Edit Just re-read Blighty post, that would do it times 25.4 out 
I do not know how you have the correct size in pixels and that resolution but get that print size.
Can you go into Image -> Print Size and check that it looks something like this, give or take a little.
![[Image: zW45X10.jpg]](https://i.imgur.com/zW45X10.jpg)
Since the printer accepts RGB images a PDF from Gimp should be ok, but still advise using Scribus. It is made for DTP and has various options for PDF creation.
I do not know how you have the correct size in pixels and that resolution but get that print size.
Can you go into Image -> Print Size and check that it looks something like this, give or take a little.
Since the printer accepts RGB images a PDF from Gimp should be ok, but still advise using Scribus. It is made for DTP and has various options for PDF creation.