M-Gallade is an underperformer in VGC. It really is. It's only purpose is to OHKO Kangakshan, chunk Venusaur / Amoonguss, and then die. Ironically, if you look at PGL statisitics, Kangaskhan is both the No. 1 Pokémon it defeats and the No. 1 Pokémon that defeats Gallade, which has to say something. A lot of people don't like their Megas being ruined by status or Intimidate, which Gallade is. Nonetheless, the determined always find a way. That's why we have science. So there must be a little merit to using him.
I can give you Gallade's biggest threats right off the tip of my tongue without even thinking. Sylveon, Salamence, Gardevoir, Aegislash, Gengar, Talonflame (don't worry about him he's awful), Charizard, anything with Scald, Landorus, Thundurus, and of course, Kangaskhan.
That's a lot of threats, but what most of them have in common are spread moves. These include Hyper Voice, Earthquake, Rock Slide, and Heat Wave. Lucky you, Gallade gets Wide Guard! So you can wall opposing Sylveon & co. while a partner takes them out.
Bisharp shares no weaknesses with Gallade, is able to completely check Sylveon, Gardevoir, somewhat check Aegislash, and possibly deal with Mence and Lando with Defiant Sucker Punches.
Milotic is good because a lot of people are going to want to Intimidate Gallade. Too bad for them. +2 Scalds and Ice Beams can eliminate a lot of threats to Gallade.
Rotom-Heat can check Charizard and Aegislash while also walling Heatran, Sylveon, most Salamence, and Thundurus. It appreciates Wide Guard support from Gallade to stop Landorus-Therian. To capitalize on this, use Will-o-Wisp or Hidden Power [Ice].
If you want to forgo Wide Guard in favor of a more offensive option, specifically another coverage move or Swords Dance, redirection is key. Amoonguss is nice because of Spore and resisting Fairy attacks. Togekiss can set up Tailwind, letting you invest more bulk into Gallade. Clefairy has Friend Guard and Helping Hand as well as Follow Me, making Gallade hit like a train. Volcrona can either support with Rage Powder + WoW or take advantage of Wide Guard to set up Quiver Dance.
Those are just a few of the partners for Mega Gallade. Of course there are more, but for the sake of time I kept it short. He appreciates Intimidate and speed control, so the genies are good partners, as well as Aegislash deterrent, which makes Heatran another solid choice. Either way, Gallade's meh potential can be made truly scary with the right partners.