Martial Gain"Victory is martial gain". That is the accepted view. However sometimes defeat can be classed as martial gain. For example if we loose a small battle we may mislead our enemy to our true strength and thus win the war. Are there really such things as defeats conferring strategic advantage? What about pretending a particular object is valuable so the enemy is tricked into competing with you for it. They triumph in the battle for the object but it is a "poison chalice" and leads them to loose out overall. Victory or defeat can constitute martial gain-it is relative to the situation, the overall context.
Depleting the enemy's morale can be seen as martial gain and yet sometimes so can boosting it. Boosting the morale of an enemy at the critical time can cause them to act in ways that lead to our overall success. Depleting the enemy's morale at the wrong time can cause them to give up an unthought out attack that would otherwise have led them to defeat.
Gain also has to do with the accumulation of good allys and having less enemies. So in this context it is not tactically intelligent to badly defeat, hurt and humiliate an opponent in a self defence situation. It is good to prove to the agressor that you are strong and can defend yourself. When you have done this and they are defeated do not brutally continue and inflict more damage to them nor humiliate them. Doing such things can have many negative repercussions and is not helpful to your spirit of compassion. You may make an enemy for life who, whilst weaker in general, may make trouble for you or surprise attack you at a later date. You may also get into trouble with the authorities and be labeled a shameful bully by observers even if the other person was the original aggressor.
If possible it is better to help the other person "save face" and be friendly or at least respectful to them. It might even be advantagous to give them a little instruction and good counsel. In this way you will be doing the good part within them benefit. You will also be hopefully gaining another ally (or at least not making an enemy). Although it might seem unlikely many times it has happened in the world that a defeated aggressor treated rightly can become a friend and ally. It has happened throughout history that such allys have on occasion saved the day and been of great assistance in later conflicts with other aggressors.
Another way that we may gain martially even if we are defeated is if we gain a new deeper understanding of martial principles and and natural compassion/self confidence. It might be better to be defeated and learn a useful insight rather than to be victorious and yet come away having been misled by a the situation and so have a distorted view. For example in general a particular technique might be useless and risky to yourself. But if you used that technique and were by some freak chance successful with it it might lead you to have an unjustified confidence in it. This could lead to disaster in future conflicts.
So "martial gain" like other concepts is much deeper than most people may appreciate. It is worthy of sincere contemplation and study.