The advantages aren't huge, but they do exist.
#1:
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252 Atk Mega Gallade Zen Headbutt vs. 252 HP / 252+ Def Clefable: 138-163 (35 - 41.3%) -- 75% chance to 3HKO after Leftovers recovery
252+ Atk Pure Power Mega Medicham Zen Headbutt vs. 252 HP / 252+ Def Clefable: 210-247 (53.2 - 62.6%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Gallade Close Combat vs. 252 HP / 252+ Def Quagsire: 187-222 (47.4 - 56.3%) -- 30.5% chance to 2HKO after Leftovers recovery
252+ Atk Pure Power Mega Medicham High Jump Kick vs. 252 HP / 252+ Def Quagsire: 310-366 (78.6 - 92.8%) -- guaranteed 2HKO after Leftovers recovery
Unaware Clefable can switch in on Gallade constantly throughout the match, while it cannot at all with Medicham. Gallade can't SD to overcome this, because of the nature of unaware. Quagsire is a similar case, but is more likely to be able to be killed by Gallade.
#2:
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Random assortment of mons, but you should get the jist. Some mons, like the above, outspeed Gallade (and Medicham) and OHKO. Medicham is useful for having what is basically an Extreme Speed that is 1/2 normal-type and 1/2 steel type. It's able to revenge kill stuff like Altaria at +1 speed whereas Gallade just dies.
#3:
/ Wherever initial power > eventual power
You may find yourself in the following situation: you have a Gallade/Medicham at 20% and are facing a Bronzong at 70%. You lose if you don't kill it, as its Gyro Ball is obviously going to annihilate you. In this situation, Medicham >>> Gallade, as Medicham is capable of dealing this damage and thus capable of winning the game for you, while Gallade is not. Yes, Gallade can hit hard thanks to SD, but it doesn't always get this chance.
Gallade and Medicham, despite their overall similarities, have different playstyles that allow Gallade not to 100% Medicham. If you require the immediate power over the power granted by setting up (in scenarios 1 and 3), Medicham >>> Gallade, and if you want something that can break walls
and revenge kill things with a base speed greater than 109 (scenario 2), medicham is still better. These scenarios are still rather limited; gallade is still clearly the better Pokemon. However, I think Medicham has enough to be considered for C+.