Usually when a GM and HC are fired it takes some time to hire new ones and the new GM goes into the off-season with the previous regimes scouts.
Maybe the thinking was, let Macc run the off-season with his guys, see how the Macc/Gase dynamic plays out during that time, and if it doesn't seem to work hire a new GM. At least he has more than enough time to assess the players, hire his own scouts, and start planning for the upcoming off-season. It also seems like they will bring someone in who already has a good relationship with Gase.
I know this all sounds backwards but it makes some sense.