I'm all for a law requiring that a 3D printed gun be given a unique serial number then be registered. Not that we live in the CSI world depicted on TV but you also can't apply the same expectations/rules on uncontrolled (unapproved?) manufacturing materials/processes.
You could ban them outright, but again if 3D printing becomes some wave of the future, then you will then either have to play catch-up, or the "anti-gun" crowd would stall those measures as an end run on banning the new standard. No clue how much of that is nonsense or the future...
I'm not saying we should do nothing but the easier something is to do behind closed doors the less effective it is to regulate. That doesn't mean you shouldn't try, but you should lower your expectations.
If someone ONLY wanted a handgun, that they expected to need to fire only a very limited amount of shots (1 or 2) to defend against home intrusion, how do you convince them that they should obey the law and NOT make a 3D gun if they couldn't afford a manufactured one? Or convince someone who already is prohibited from owning a legal gun that it doesn't make sense to avoid the risk of buying a gun on the street? If they are only going to use it in a life or death defensive situation, the risk of, "oh no, I'll get in trouble!" doesn't mean a lot.
I rate this along with the gun safe legislation. It's not a BAD idea, it's just not going to amount to much beyond an additional fine/time on top of other charges.
how far would "our side" have to change before they sat down in good faith?
Being well informed on the mechanics and logistics of weapons and their ownership as well as making proposals that would have significant impact on stated goals would be a good start. Then comes the hard (impossible?) ones. Acknowledging that someone has the right to defend their life with lethal force. Acknowledging that firearm collection and range shooting is a hobby / sport to some. Acknowledging that a one size fits all solution may not work when considering rural/wilderness residents compared to urban residents.
Then you need to convince some that any concessions are well meaning and reasonable on their own and not just a "what we could get push through or trick the other side into accepting because we can't achieve a full ban... yet." That is already impossible for SOME on the other side. For others, the tactics used in the name of reason or responsibility that fall short is poisoning more against you.