Not a bad spot for this question.
You can download pre existing spawn grids for some maps here:
http://radioactivecricket.com/files/pnrp_gridfiles/Once downloaded you just need to copy the txt files to this location on your server: /garrysmod/data/postnukerp/export_import
Once the files are there, you just need to go into the Admin Menu and import them. If you update or change your grid maps you can export them and they will overwrite the existing file. This is good for backing up the grids and such.
Speaking of backing up, always make sure you backup the sv.dbin the garrysmod folder on your server every now and then. This file is basically the database.