I'm going to have to say Flygon
The additional Ice, Dragon and Fairy weakness flygon bring are easily handled by your two steel types. In addition, the rock resistance in addition to the ground and electric immunities are high appreciated, especially by Charizard. Gourgeist's multitude of weaknesses, most notably to fire, dark and ghost, and the burden of finding the size of gourgeist you desire and evolving it via trade, make Flygon even more appealing. And of course, Flygon also adds a little more speed to your team.
A moveset could look like:
Flygon @ Dragon Fang/Soft Sand
Steel Wing, Fire Blast and U-Turn are also fine move choices.
Mawile Looks fine, but I advise that this be your mega-evolution, as it gains a massive amount of power, more than any other mega really, when it mega evolves
Qwilfish: Qwilfish's physical attack is much higher than his special. As such, I would suggest waterfall over surf, and perhaps Payback, Fell Stinger or another physical move over ice beams. Spikes/Toxic Spikes may also be used.
Goodra: Fine. You may run surf on this if you choose not to on Qwilfish
Aegislash: Night Slash is pointless, as it gets essentially the same coverage as shadow sneak. Try King's Shield instead. You may run shadow ball or flash cannon if you desire, as aegislash has a huge base 150 special attack in blade form.
Hope this helps!