Okay, my take.
Diana deals with the baby's death by refusing to accept it, and spends the next sixteen years watching her son grow up, etc.
Dan deals with the baby's death by denying there ever was a son. He recognizes that Diana has a mental problem caused by the death, but never, until the end of the play, accepts that his way of dealing/not dealing is also a form of mental illness.
There are some clues along the way, as at the end of I AM THE ONE in Act 1, the very last line is Gabe's, directed at Dan: "You just don't know who I am". Also at one point I believe Gabe says something like "I've never heard you say my name" which sets up that final scene between Dan and Gabe. No, I don't believe that Dan "sees" Gabe, but finally is acknowledging that there WAS a Gabe, and accepts the doctor's offer as he realizes he will need some help in adjusting to a fact that denied for so long.
Just my take, however ....
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