This Cyclingtips podcast contains an interview with Ruckus Composites, which repairs carbon frames. To be honest, I listened to it a week ago and I don't recall the details of the contents. I do recall that there is a lot of thought that goes into properly repairing any structural carbon damage. They have to consider how much to reinforce the damaged area, and with what types of carbon (e.g. bikes may contain fiber of different moduli). They need to work with the layup of the fibers, e.g. what orientation the fibers are in. They ultrasound the frame to determine the extent of the damage, then they decide how much to sand off. Carbon fiber dust can damage your lungs, so I think this operation needs to be done with respirators. Contamination can result in the epoxy not bonding properly and failing later on, so this should be done in a clean room. To hear Ruckus tell it, there was also somewhat of a learning curve with carbon repair. Basically, I think there are some very good reasons not to try this yourself.
The podcast did say that carbon is much easier to repair than metal. You can repair just the damaged area. If a metal tube took structural damage, a repairer has to un-bond the tubes from their neighboring joints, then weld a whole new tube in. If the frame is aluminum, they then have to heat treat the whole frame (this bit means that aluminum repairs are technically possible but rarely cost effective).
I want to be very cautious about saying this: while there are excellent reasons not to repair this yourself, you clearly feel that the damage is at the surface only. This post on Singletracks.com shows a DIY repair of what the poster believed to be a bad surface gouge. I lack actual experience with composite repair, but some critiques rise to mind immediately: we don't know how sensitive that epoxy is to the curing temperature or contamination, and if the OP didn't control those factors, that repair could fail later. Nevertheless, that was a non-structural repair, so it's possible that the bike would be unaffected even if the repair delaminated. That repair kit was purchased on Amazon. So, there are over the counter kits available.
With your bike, the damage is deep enough that I wonder if any of the actual carbon fibers have been torn/abraded. This is what Argenti's first sentence alludes to. Recall that carbon bikes are made from little sheets of fiber like the ones in the repair kit linked above, and those are wrapped around a mandrel. In the bike industry, I believe most or all the carbon sheets are already impregnated with resin, but that resin is the balance of the structure. If you scrape the surface of a carbon structure, that's not so bad, but if any fibers are torn, I would send it to a professional. Also consider that there may be damage you can't see. Since you don't recall how the bike got that way, it's possible it was an impact rather than just a scrape. And that area is probably one of the most highly stressed areas on the bike.
In any case, many current gravel and mountain bikes have rubber bumpers in the area around the down tube. You can also buy protective frame tape. Pinkbike reviewed 7 brands of protective tape, and more brands exist. You may wonder how a thin layer of tape can possibly protect against impacts. The answer is more than you might think, although obviously tape alone can't ward off a massive impact to a tube.
