The desire to train your "inner chest" to fill the gap between your pecs usually leads to recommendations of doing certain exercises which "focus on the inner chest".
To some extent, there is evidence that a muscle isn't activated the same amount along its entire length and that different parts of muscles can grow more depending on how much they are activated, though these differences are likely not relevant as they are so small.
When it comes to the chest, however, it is important to realize that people with a "chest gap" have their chest muscles (called m. pectoralis major) attaching further out on their breast bone (called the sternum). This attachment cannot be moved by any exercise and is determined genetically.
Thus the most that you can hope to accomplish if you have a chest gap is to train your chest to grow to make the gap less noticeable, but the gap will always remain there. This shouldn't discourage you though! A great physique is determined by so much more than the symmetry of your chest, so work hard and build the best version of YOUR body!
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THE EVIDENCE: Wakahara T. Association between regional differences in muscle activation in one session of resistance exercise and in muscle hypertrophy after resistance training. Eur J Appl Physiol. 2012 Apr;112(4):1569-76.