150-180ppi should give you good visual quality for testing. I am not familiar with tiling in gimp but you could always layout some guides diving the map into print-sized parts with a tiny registration dot where the guides cross, then put a guide about .25" to .5" (1cm) on either side of those guides to mark desired overlap (makes it easier to reassemble without ugly white gaps)... then copy and paste each of those sections (including a section for overlap) into their own document for print. Hope that makes sense?!
If you have access to Adobe Illustrator.. you can place the map and set it to tile in page setup.