Gimp tutorial on how to make scanlines in GIMP by making a simple pattern.
If you want you lines further apart make the tiny image bigger, like 6×6 pixels or even larger. And same goes for scanlines closer together, make the image 3×3 pixels.
I’ve also heard them referred to as stripes in GIMP, lines in GIMP, diagonal blinds in GIMP, diagonal stripes in GIMP.