Presuming this is already prepped, you’re going to want to make it greyscale and up the contrast if you’re going to engrave it. It should look too harsh to the eye when it’s right for the laser. There’s a great thread on here going through the entire process for photoshop - but you call follow along in GIMP you just need to find the equivalent commands.
Once you’re there, the convert to dots or convert to patterns under SD Graphic does a fabulous job on images!