Ah but I'm not thinking of any light. Just a thin light plastic plate mounted directly onto the fender. At one time one might have used a stick on plate, just wrapped around the curve of the fend... sorry, mudguard. Let's stick to UK English
Now I'm thinking of a rigid plate, but no need for any additional mounting hardware, so shouldn't be enough weight to cause the mudguard to flex too much and break.
If I did feel the need for a licence plate light, one can get very small LED items that again would add no significant weight. It's all just an intellectual exercise at the moment, until I get hold of a bike.
BTW, the 'gravel guard' as I think you are referring to the big arm that mounts to the swing arm and has the licence plate holder and light, is not actually that. The manufacturer cares little about how much gets thrown up the rider's back, but it does have to comply with the regs and they state that the rear tyre cannot be the rearmost part of the bike. There has to be something else, behind the tyre to comply. So many manufacturers do their fancy styling with silly short mudguards (that may indeed look cool) and then mount a big arm off the swingarm that ensures the bike complies and often they'll stick the license plate on there too. But it's not through any altruistic desire to keep you clean. They do it because they have to.
The alternative is to use a short mudguard from the Icon or Urban Enduro and then a tail tidy to use the plate as an extension to reduce the amount of cack thrown up, but I do like the Al 'guards of the Classic and really do NOT like the yucky green of the UE. Sorry if anyone likes that colour, but I really do not. Otherwise the UE might just be a better starting point.
So, my Classic might look a bit like this, although no actual plate is shown and I'm torn over using FT 'bars. I think they look better and on the road I found them more comfortable for me, but I want to do some (gentle) off roading and the higher ones might be better. So jury's still out on that.