When I import my design into GF, it only allows me to CUT the two lines I added in Inkscape. When I try to choose “Engrave” they disappear(?) How can I fix this? I even tried adding two lines using the GF software, and they STILL disappear when I choose “Engrave”.
Vector lines will score or cut. In Inkscape you need to thicken the line or make a very narrow rectangle foe an engrave operation. Depending on how thick you want your lines to appear, a defocused score operation might work.
A single line parallel to the direction of the laser will not engrave much of anything, There has to be fill somewhere or there is nothing to engrave. If you use Stroke to Path on thickened lines then there is fill that can be engraved
You can get wider lines by engraving them. Set your stroke width the way you want it to look, then do Path->stroke to path. This will convert them to engravable shapes.
The lines added in the GF are vectors so they will score or cut. You could use the interface to make a very thin rectangle that would engrave, but I think making the line thicker in Inkscape is the better option.