Gimp works in pixels, and that information is hidden in your screenshot by the window icons. That will determine the final quality.
(1) There is a script-fu that will place a guide using "real-world" units. Only one guide at a time, use multiples of 21 cm and 29.7 and build up the grid for Image -> Slice using Guides. The script shows in the Image -> Guides -> New Guide (by unit)
(attached, unzip and put in your scripts folder)
(2) Another option is the Rectangular Selection tool with the aspect ratio set to 1:1.414 Make selection then Image -> Guides -> New Guides from Selection. Move the selection around and build up the grid of guides.
(1) There is a script-fu that will place a guide using "real-world" units. Only one guide at a time, use multiples of 21 cm and 29.7 and build up the grid for Image -> Slice using Guides. The script shows in the Image -> Guides -> New Guide (by unit)
(attached, unzip and put in your scripts folder)
(2) Another option is the Rectangular Selection tool with the aspect ratio set to 1:1.414 Make selection then Image -> Guides -> New Guides from Selection. Move the selection around and build up the grid of guides.