Once a father took his ten year old son to a playground. He threw a ball and the game is that the boy has to bring the ball back to his father. The ground is full of toys. On the way, the boy gets distracted by a toy and starts playing with it instead, till his father shouts to remind him about the ball. He leaves the toy and starts running after the ball again.
There are other kids in the playground. This time the boy finds another attractive toy and starts playing with it. The boy starts crying when another strong kid comes and snatches that toy. Next time, the boy himself snatches a toy from another smaller kid. Throughout the journey, fights keep erupting for toys. All this while the father is standing just behind the son. But for the boy who got lost in the toys, his father is so near yet so far.
This story helps us to understand when Krishna says, "He is within and without all that exists, the unmoving and also the moving; That is incomprehensible (𝙖𝙫𝙞𝙟𝙣𝙚𝙮𝙖𝙢) due to subtlety and That is far and near" (13.16). Like the father in the above story, HE is just behind us throughout our life's journey and we just have to look back. When we get lost in attachments, HE helps us by sending various experiences; giving us tough situations to remind us like the father shouting at the boy.
When Krishna says HE is incomprehensible, the indication is that he is incomprehensible to our senses because of their limitations. He can be attained through experiences but not through explanations. For someone who never tasted salt or sugar, no amount of explanation would help to understand their taste. The only way to comprehend them is to taste them i.e. experience them with awareness.