The scanline gap doesn't actually come from the PCE itself, or its signal in particular. Rather it comes from the televisions having a fixed frequency scanrate, coupled with an expected interlaced display.
The gap between the scanlines, that look, has many contributing factors because TV sets back in the day were garbage/low quality compared to RGB monitors. I'm referring to the beam actually widening (in both directions, but specifically vertically). That's why the gaps might not appear on bright areas or as strong (whites and such), but do on lower luma colors. It's a blooming effect on the beam. Secondly, interlace displays have interlacing artifacts (combing artifacts), so the TV manufacture purposely inflate the height of the scanling being drawn so the overlap would hide interlacing (combined with the blooming effect for bright colors).
So your scanline experience would vary from model, to brand, or your personally display settings. The TV I bought in my teen years to play my 16bit systems, had a nice clean/clear scanline gap effect. Arcade monitors BITD had more of pronounced scanline gap effect because the beams were sharper/narrower focused. Generally, the better the TV set the more pronounced the scanline gap would be.
