While having Retaliate on a scarf user is a viable strategy, I don't think Gallade is a particularly good user of this strategy. Gallade is usually more about bulk and its ability to either use Bulk Up or switch between a variety of coverage moves (Drain Punch, Psycho Cut, Leaf Blade, Stone Edge, Shadow Sneak etc.). Scarfing it would make it a one trick pony and allow your opponent to force you to switch Gallade out after it used Retaliate. A competent opponent would predict that it was scarfed, and possibly switch in a ghost/ steel type as well.
So my personal opinion is that Gallade has better things to do with its moveset and item than using a Retaliate Scarf set.
The best user for this strategy is usually Slaking, because of Truant it can't stay in for more than 1 turn anyway, and bar Ghost types, nothing wants a STAB Retaliate coming from Slaking to hit them.