The point of banning guns isn't to ban guns. It's to reduce and minimize the associated gun violence. If you were to go by your compromise, what has actually been achieved in this regard? Meanwhile if you were to examine banning all abortions, if your goal is to prevent "lives" lost via abortions, banning all abortions is a fairly complete solution. Value wise, they aren't equal in the slightest.
If we were to go further and you could offer a complete solution for gun violence in return for a complete abortion ban, from a moral perspective, they would be still unequal in value. Even if you justify banning abortions as saving innocent babies, there's no way to get around the fact that you are trampling on female rights to bodily autonomy to do so. With gun control, I don't believe there is much moral reasoning in being against gun control.
Finally, such a compromise would be stupid because both parties aren't completely black and white on such issues. Not all democrats wish to ban all guns. And not all republicans wish to ban all abortions. A good compromise is one that pleases the most people. Banning all abortions is incredibly unpopular even among republicans