Don't worry I had no idea idea when I started using GIMP. I used to draw a grid in crazy ways until I found the function under Pattern.
My Encounter Maps: Madness at Gardmore Abbey; Assault on Nightwyrm Fortress; Orcus Conversion; Castle Jacktannery; Small buildings; Dwarf Goldmine
Here's a video I made on the basics of grids using GIMP. This video also shows you how to make the little "cross" markers so popular in outdoor 4e battlemaps
Gimp basics - YouTube