It depends. Gallade is a solid Pokémon, with a great attack stat and an ok special defense stat. And it can learn good coverage moves. However, dawn stones are not easily obtained. The only guaranteed way to get your hands on one is to beat Wally on Victory Road. He will give one to you as a reward for beating him, with the wish that you will use it to evolve a male Kirlia into Gallade. If you want to use Mega Gallade, the Mega Stone is not available until completing the Delta Episode in the Post-Game, as a gift from Professor Cosmo in thanks for saving the world. Other ways to get a Dawn Stone: Grind Inverse Battles in hopes that you will get one as a reward, by having a Rematch with Mysterious Sisters Scall and Ion in Sea Mauville (that is probably Post-Game), or hope you get one from a Secret Pal using the "Pick Up Stones" skill. Looking at his viability on the gameplay side of things, he will be super effective agianst: May/Bredan's Magcargo, Breloom, and Blaziken (all depending on your starter choice) (He could also be super effective against their Wailord if he has Leaf Blade, but I digress), Wally's Delcatty, Roselia, Gallade, and Magneton, Brawly's entire team as a Ralts/Kirlia, Norman's entire team if he is evolved into Gallade, Sidney's entire team, Glacia's entire team except her 2 Froslass, and Steven's Cradily and Aggron. He is weak against Winnona's Gym, May/Brendan's Swellow, Tate and Liza's Gym, Pheobe's entire team, Wally's Altaria and Gallade, and Steven's Metagross and Skarmory. So here is a quick list of pros and cons to finish up my extremely long winded answer.
Pros:
-Sky-High 125 Base Atk
-Helpful with many key battles
-Has a great type/STAB Combo
-Decent movepool
-Pre-evolution obtainable early
Cons:
-Hard to get evolution Item to evolve him
-Will be outspeeded a decent amount
-Powerful Mega Evolution not obtainable until after the Post-Game is complete
So to conclude, Gallade is great, but is hard to get. If you have acess to trading, try to find someone who give you a Dawn Stone, (and if they are generous, a Galladite) and you could have a Gallade on your team as soon as your Ralts evolves at Lvl 20.
