I calculate for desirable surface temperatures, the planet's mass needs to be close to 40% of the Earth's mass in order to retain Oxygen-Nitrogen with gaseous water over geological time. A colder surface temperature can lower that mass somewhat (perhaps down to 35%) but you're talking about permanent Arctic like conditions.
However, the collision of these two bodies will generate a great deal of heat. That will be enough to melt vast portions of the combined planet. In fact, it'll probably vaporize most the water provided by Ganymede and some will be lost to space. It is likely the surface will remain uninhabitable due to the high temperatures for many thousands to a million years or so.
Even worse, the plan suffers from another problem. Assume everything else works and you get a planet with the proper temperature, mass, etc. Ganymede possesses so much water (about 50x as much as on the Earth). Earth's water covers its surface to an average depth of 2,700 m or 2.7 km. Ganymede's water will cover Mars to an average depth of 521 km and will cover everything - including Olympus Mons.
So good news, you may remelt Marmede's core and generate a magnetic field. Bad news, don't expect to move in for the likely life of the human species and when you can move in, you'd better be sporting flippers.