There has been some recent discussion about what it means to gap fill in martial arts, and some fundamental misunderstanding of the terms used.
Filling gaps in understanding happens when a system has not been transmitted correctly and is "broken", or non functional. An example would be wing chun that works only at an artificial "chi sau range". The gap in question in this case is the whole free fighting aspect of the system, i.e. understanding has only been transmitted up to chi sau level with a corresponding misunderstanding of what chi sau is for. The system in this case is completely broken, both in terms of functionality at close range (trying to fight with chi sau) and at longer range (unable to do anything but try to re-establish chi sau range).
Gap-filling suggestions from people with this kind of broken understanding range from filling the gap with other incompatible systems like boxing or Muay Thai, to simply imagining that chi sau range will be maintained as in the hi sau drill, whether the opponent likes it or not. Obviously this kind of gap filling can lead to problems in terms of functionality.
Gap filling within a system is not comparable to cross training in a different system. Cross training can be a good idea with compatible systems, and a bad idea with incompatible ones, again for obvious reasons.
Opinions please